Add note about setting proxy for bootstrap.
Add note about setting proxy for bootstrap.

Users may not expect that bootstrap.sh goes out and fetches things from
the internet. If internet access requires a proxy, they won't know that
they need to set up the proxy environment variables before running
bootstrap.

We can't tell them exactly how to set up their proxy, but they should
know how to do that if it's required. We just need to make them aware
that bootstrap requires internet access.

Fixes #473

tjyang/vitess,kuipertan/vitess,kmiku7/vitess-annotated,michael-berlin/vitess,ptomasroos/vitess,mlc0202/vitess,dumbunny/vitess,alainjobart/vitess,yangzhongj/vitess,fengshao0907/vitess,ptomasroos/vitess,cloudbearings/vitess,anusornc/vitess,kuipertan/vitess,AndyDiamondstein/vitess,mapbased/vitess,anusornc/vitess,erzel/vitess,kuipertan/vitess,sougou/vitess,mlc0202/vitess,applift/vitess,netroby/vitess,erzel/vitess,yangzhongj/vitess,pivanof/vitess,skyportsystems/vitess,ptomasroos/vitess,michael-berlin/vitess,netroby/vitess,cgvarela/vitess,vitessio/vitess,tirsen/vitess,atyenoria/vitess,tinyspeck/vitess,erzel/vitess,tirsen/vitess,netroby/vitess,SDHM/vitess,tinyspeck/vitess,yangzhongj/vitess,netroby/vitess,yaoshengzhe/vitess,nurblieh/vitess,xgwubin/vitess,mlc0202/vitess,AndyDiamondstein/vitess,tirsen/vitess,enisoc/vitess,dcadevil/vitess,tjyang/vitess,applift/vitess,mapbased/vitess,davygeek/vitess,mapbased/vitess,michael-berlin/vitess,kuipertan/vitess,aaijazi/vitess,erzel/vitess,SDHM/vitess,yaoshengzhe/vitess,mattharden/vitess,yaoshengzhe/vitess,mapbased/vitess,cgvarela/vitess,pivanof/vitess,guokeno0/vitess,SDHM/vitess,ptomasroos/vitess,vitessio/vitess,pivanof/vitess,AndyDiamondstein/vitess,alainjobart/vitess,tirsen/vitess,atyenoria/vitess,cloudbearings/vitess,atyenoria/vitess,atyenoria/vitess,AndyDiamondstein/vitess,yangzhongj/vitess,netroby/vitess,mahak/vitess,cgvarela/vitess,applift/vitess,mattharden/vitess,mattharden/vitess,sougou/vitess,dumbunny/vitess,guokeno0/vitess,kmiku7/vitess-annotated,cloudbearings/vitess,dumbunny/vitess,yaoshengzhe/vitess,cgvarela/vitess,yaoshengzhe/vitess,aaijazi/vitess,fengshao0907/vitess,mlc0202/vitess,kmiku7/vitess-annotated,mahak/vitess,mahak/vitess,mattharden/vitess,rnavarro/vitess,enisoc/vitess,mattharden/vitess,tjyang/vitess,michael-berlin/vitess,mattharden/vitess,xgwubin/vitess,aaijazi/vitess,mapbased/vitess,xgwubin/vitess,mlc0202/vitess,HubSpot/vitess,SDHM/vitess,HubSpot/vitess,tinyspeck/vitess,rnavarro/vitess,xgwubin/vitess,tjyang/vitess,kuipertan/vitess,fengshao0907/vitess,HubSpot/vitess,tjyang/vitess,guokeno0/vitess,enisoc/vitess,vitessio/vitess,kuipertan/vitess,ptomasroos/vitess,aaijazi/vitess,netroby/vitess,pivanof/vitess,SDHM/vitess,dcadevil/vitess,HubSpot/vitess,mahak/vitess,SDHM/vitess,cgvarela/vitess,HubSpot/vitess,enisoc/vitess,mapbased/vitess,tirsen/vitess,atyenoria/vitess,nurblieh/vitess,cgvarela/vitess,nurblieh/vitess,nurblieh/vitess,mahak/vitess,nurblieh/vitess,guokeno0/vitess,applift/vitess,vitessio/vitess,skyportsystems/vitess,applift/vitess,mattharden/vitess,michael-berlin/vitess,davygeek/vitess,kmiku7/vitess-annotated,dcadevil/vitess,cloudbearings/vitess,mapbased/vitess,tirsen/vitess,AndyDiamondstein/vitess,davygeek/vitess,aaijazi/vitess,nurblieh/vitess,tjyang/vitess,kmiku7/vitess-annotated,sougou/vitess,alainjobart/vitess,netroby/vitess,applift/vitess,cloudbearings/vitess,mlc0202/vitess,cloudbearings/vitess,yaoshengzhe/vitess,pivanof/vitess,xgwubin/vitess,applift/vitess,sougou/vitess,applift/vitess,SDHM/vitess,xgwubin/vitess,alainjobart/vitess,pivanof/vitess,fengshao0907/vitess,tjyang/vitess,AndyDiamondstein/vitess,davygeek/vitess,guokeno0/vitess,guokeno0/vitess,tjyang/vitess,guokeno0/vitess,yangzhongj/vitess,kmiku7/vitess-annotated,AndyDiamondstein/vitess,vitessio/vitess,dcadevil/vitess,tinyspeck/vitess,skyportsystems/vitess,michael-berlin/vitess,yangzhongj/vitess,atyenoria/vitess,pivanof/vitess,tirsen/vitess,mattharden/vitess,pivanof/vitess,mlc0202/vitess,rnavarro/vitess,kuipertan/vitess,cloudbearings/vitess,dumbunny/vitess,yaoshengzhe/vitess,HubSpot/vitess,alainjobart/vitess,erzel/vitess,dumbunny/vitess,skyportsystems/vitess,rnavarro/vitess,davygeek/vitess,alainjobart/vitess,michael-berlin/vitess,applift/vitess,dumbunny/vitess,HubSpot/vitess,vitessio/vitess,davygeek/vitess,atyenoria/vitess,erzel/vitess,sougou/vitess,atyenoria/vitess,dcadevil/vitess,HubSpot/vitess,tirsen/vitess,dcadevil/vitess,sougou/vitess,fengshao0907/vitess,erzel/vitess,erzel/vitess,dumbunny/vitess,yaoshengzhe/vitess,nurblieh/vitess,mattharden/vitess,nurblieh/vitess,michael-berlin/vitess,netroby/vitess,dumbunny/vitess,tinyspeck/vitess,xgwubin/vitess,fengshao0907/vitess,michael-berlin/vitess,yangzhongj/vitess,rnavarro/vitess,kmiku7/vitess-annotated,anusornc/vitess,tjyang/vitess,guokeno0/vitess,cloudbearings/vitess,enisoc/vitess,skyportsystems/vitess,aaijazi/vitess,xgwubin/vitess,tirsen/vitess,kmiku7/vitess-annotated,anusornc/vitess,ptomasroos/vitess,applift/vitess,tinyspeck/vitess,rnavarro/vitess,guokeno0/vitess,mapbased/vitess,rnavarro/vitess,netroby/vitess,mattharden/vitess,anusornc/vitess,cgvarela/vitess,dumbunny/vitess,yangzhongj/vitess,erzel/vitess,cgvarela/vitess,fengshao0907/vitess,anusornc/vitess,aaijazi/vitess,mlc0202/vitess,ptomasroos/vitess,SDHM/vitess,anusornc/vitess,atyenoria/vitess,ptomasroos/vitess,rnavarro/vitess,kuipertan/vitess,fengshao0907/vitess,yaoshengzhe/vitess,cloudbearings/vitess,erzel/vitess,skyportsystems/vitess,davygeek/vitess,skyportsystems/vitess,dumbunny/vit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