Update Ubuntu 18.04 build instructions
Update Ubuntu 18.04 build instructions

Update Ubuntu 18.04 build instructions to include OpenSSL 1.0.2 headers

https://github.com/dotnet/corefx/issues/28924#issuecomment-379889142
wtgodbe/corefx,Jiayili1/corefx,Jiayili1/corefx,mmitche/corefx,BrennanConroy/corefx,ptoonen/corefx,shimingsg/corefx,ViktorHofer/corefx,shimingsg/corefx,wtgodbe/corefx,mmitche/corefx,ptoonen/corefx,ptoonen/corefx,wtgodbe/corefx,mmitche/corefx,BrennanConroy/corefx,mmitche/corefx,shimingsg/corefx,ericstj/corefx,Jiayili1/corefx,Jiayili1/corefx,ericstj/corefx,ViktorHofer/corefx,BrennanConroy/corefx,mmitche/corefx,wtgodbe/corefx,shimingsg/corefx,shimingsg/corefx,wtgodbe/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ptoonen/corefx,Jiayili1/corefx,ptoonen/corefx,Jiayili1/corefx,shimingsg/corefx,ericstj/corefx,ptoonen/corefx,ViktorHofer/corefx,mmitche/corefx,ptoonen/corefx,ericstj/corefx,wtgodbe/corefx,Jiayili1/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,mmitche/corefx,wtgodbe/corefx