minor #28543 Added the Code of Conduct file (javiereguiluz)
minor #28543 Added the Code of Conduct file (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #28543).

Discussion
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Added the Code of Conduct file

Symfony already has a Code of Conduct (https://symfony.com/coc) but we haven't add it to the repository. This makes others think that Symfony doesn't have a CoC. See for example https://github.com/symfony/symfony/community

![community-profile](https://user-images.githubusercontent.com/73419/45891994-362f2100-bdc7-11e8-8870-45e8383ce61a.png)

The contents of this PR are the exact same contents given by GitHub when clicking on "Add a Contributor Covenant based CoC".

Commits
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06295442d1 Added the Code of Conduct file

ragboyjr/symfony,MatTheCat/symfony,mweimerskirch/symfony,shieldo/symfony,pierredup/symfony,zerkms/symfony,gnat42/symfony,chalasr/symfony,ogizanagi/symfony,Deamon/symfony,phramz/symfony,smatyas/symfony,mnapoli/symfony,SaschaDens/symfony,alekitto/symfony,jvasseur/symfony,Taluu/symfony,localheinz/symfony,bendavies/symfony,ro0NL/symfony,smatyas/symfony,jakzal/symfony,vincentaubert/symfony,mnapoli/symfony,tucksaun/symfony,ragboyjr/symfony,damienalexandre/symfony,Simperfit/symfony,jderusse/symfony,Taluu/symfony,ajgarlag/symfony,ro0NL/symfony,d-ph/symfony,frankdejonge/symfony,zerkms/symfony,maidmaid/symfony,sroze/symfony,andreia/symfony,ragboyjr/symfony,derrabus/symfony,sroze/symfony,vincentaubert/symfony,smatyas/symfony,d-ph/symfony,pierredup/symfony,MatTheCat/symfony,Koc/symfony,jvasseur/symfony,Nyholm/symfony,oleg-andreyev/symfony,Simperfit/symfony,shieldo/symfony,zorn-v/symfony,sroze/symfony,docteurklein/symfony,jderusse/symfony,mnapoli/symfony,michaelperrin/symfony,frankdejonge/symfony,Deamon/symfony,romaricdrigon/symfony,hhamon/symfony,docteurklein/symfony,michaelperrin/symfony,nicolas-grekas/symfony,jakzal/symfony,deguif/symfony,joelwurtz/symfony,docteurklein/symfony,mweimerskirch/symfony,mweimerskirch/symfony,nicolas-grekas/symfony,zcodes/symfony,joelwurtz/symfony,OndraM/symfony,voronkovich/symfony,Deamon/symfony,MatTheCat/symfony,zcodes/symfony,Tobion/symfony,garak/symfony,arjenm/symfony,bendavies/symfony,lyrixx/symfony,d-ph/symfony,ro0NL/symfony,joelwurtz/symfony,frankdejonge/symfony,phramz/symfony,michaelperrin/symfony,Nyholm/symfony,sgehrig/symfony,OskarStark/symfony,localheinz/symfony,pyrech/symfony,andreia/symfony,nicolas-grekas/symfony,1ed/symfony,xabbuh/symfony,mnapoli/symfony,Nyholm/symfony,Deamon/symfony,SaschaDens/symfony,Koc/symfony,mnapoli/symfony,pyrech/symfony,fabpot/symfony,maidmaid/symfony,stof/symfony,tgalopin/symfony,OskarStark/symfony,OndraM/symfony,michaelperrin/symfony,ogizanagi/symfony,1ed/symfony,sgehrig/symfony,frankdejonge/symfony,Slamdunk/symfony,teohhanhui/symfony,Simperfit/symfony,ajgarlag/symfony,HeahDude/symfony,damienalexandre/symfony,lstrojny/symfony,joelwurtz/symfony,romaricdrigon/symfony,gonzalovilaseca/symfony,andrerom/symfony,hhamon/symfony,Koc/symfony,alekitto/symfony,Tobion/symfony,zcodes/symfony,tgalopin/symfony,arjenm/symfony,tucksaun/symfony,paradajozsef/symfony,jakzal/symfony,localheinz/symfony,SaschaDens/symfony,stof/symfony,mpdude/symfony,ragboyjr/symfony,mweimerskirch/symfony,Koc/symfony,OskarStark/symfony,1ed/symfony,mpdude/symfony,voronkovich/symfony,damienalexandre/symfony,teohhanhui/symfony,gnat42/symfony,1ed/symfony,yceruto/symfony,maidmaid/symfony,oleg-andreyev/symfony,SaschaDens/symfony,gonzalovilaseca/symfony,Taluu/symfony,symfony/symfony,yceruto/symfony,sgehrig/symfony,teohhanhui/symfony,andrerom/symfony,andreia/symfony,chalasr/symfony,stof/symfony,garak/symfony,zcodes/symfony,docteurklein/symfony,xabbuh/symfony,curry684/symfony,tucksaun/symfony,pierredup/symfony,kbond/symfony,andrerom/symfony,Slamdunk/symfony,zorn-v/symfony,Tobion/symfony,bendavies/symfony,stof/symfony,pierredup/symfony,mpdude/symfony,andreia/symfony,gonzalovilaseca/symfony,yceruto/symfony,romaricdrigon/symfony,vincentaubert/symfony,Slamdunk/symfony,voronkovich/symfony,kbond/symfony,alekitto/symfony,ogizanagi/symfony,zerkms/symfony,romaricdrigon/symfony,HeahDude/symfony,localheinz/symfony,HeahDude/symfony,Deamon/symfony,gnat42/symfony,jderusse/symfony,zerkms/symfony,dmaicher/symfony,bendavies/symfony,curry684/symfony,deguif/symfony,phramz/symfony,jvasseur/symfony,lyrixx/symfony,dmaicher/symfony,teohhanhui/symfony,jderusse/symfony,yceruto/symfony,curry684/symfony,MatTheCat/symfony,maidmaid/symfony,garak/symfony,zorn-v/symfony,dmaicher/symfony,paradajozsef/symfony,deguif/symfony,OndraM/symfony,sroze/symfony,xabbuh/symfony,zorn-v/symfony,damienalexandre/symfony,tucksaun/symfony,shieldo/symfony,lyrixx/symfony,symfony/symfony,ogizanagi/symfony,lstrojny/symfony,hhamon/symfony,alekitto/symfony,pyrech/symfony,pyrech/symfony,dmaicher/symfony,d-ph/symfony,smatyas/symfony,shieldo/symfony,deguif/symfony,derrabus/symfony,gonzalovilaseca/symfony,tgalopin/symfony,oleg-andreyev/symfony,arjenm/symfony,chalasr/symfony,jakzal/symfony,chalasr/symfony,oleg-andreyev/symfony,voronkovich/symfony,andrerom/symfony,derrabus/symfony,smatyas/symfony,fabpot/symfony,vincentaubert/symfony,paradajozsef/symfony,jvasseur/symfony,docteurklein/symfony,symfony/symfony,ro0NL/symfony,OndraM/symfony,tgalopin/symfony,Taluu/symfony,nicolas-grekas/symfony,HeahDude/symfony,fabpot/symfony,Simperfit/symfony,garak/symfony,arjenm/symfony,Slamdunk/symfony,xabbuh/symfony,Nyholm/symfony,symfony/symfony,kbond/symfony,Tobion/symfony,ajgarlag/symfony,lyrixx/symfony,SaschaDens/symfony,kbond/symfony,OndraM/symfony,mweimerskirch/symfony,mpdude/symfony,gnat42/symfony,phramz/symfony,sgehrig/symfony,paradajozsef/symfony,fabpot/symfony,hhamon/symfony,derrabus/symfony,Simperfit/symfony,lstrojny/symfony,ajgarlag/symfony,OskarStark/symfony,curry684/symfony