Install.rst: Add pip upgrade info
Install.rst: Add pip upgrade info

This commit addresses the fix for error
caused due to using outdated PIP, in Install docs.

Fixes https://github.com/coala-analyzer/coala/issues/2124

RJ722/coala,tushar-rishav/coala,Asnelchristian/coala,ManjiriBirajdar/coala,jayvdb/coala,sagark123/coala,sils1297/coala,coala-analyzer/coala,stevemontana1980/coala,damngamerz/coala,ayushin78/coala,MariosPanag/coala,incorrectusername/coala,jayvdb/coala,yashLadha/coala,RJ722/coala,scottbelden/coala,aptrishu/coala,coala/coala,ayushin78/coala,SanketDG/coala,rimacone/testing2,aptrishu/coala,arush0311/coala,impmihai/coala,SanketDG/coala,RJ722/coala,ManjiriBirajdar/coala,scottbelden/coala,nemaniarjun/coala,tltuan/coala,JohnS-01/coala,mr-karan/coala,netman92/coala,shreyans800755/coala,arush0311/coala,coala/coala,tltuan/coala,ManjiriBirajdar/coala,meetmangukiya/coala,tltuan/coala,vinc456/coala,d6e/coala,impmihai/coala,tushar-rishav/coala,karansingh1559/coala,MariosPanag/coala,karansingh1559/coala,abhiroyg/coala,Asalle/coala,shreyans800755/coala,Asalle/coala,Balaji2198/coala,Asnelchristian/coala,NiklasMM/coala,CruiseDevice/coala,yashLadha/coala,refeed/coala,arjunsinghy96/coala,CruiseDevice/coala,rimacone/testing2,ayushin78/coala,yashLadha/coala,d6e/coala,mr-karan/coala,MariosPanag/coala,Nosferatul/coala,coala-analyzer/coala,aptrishu/coala,Asalle/coala,d6e/coala,Nosferatul/coala,sagark123/coala,netman92/coala,incorrectusername/coala,refeed/coala,karansingh1559/coala,Balaji2198/coala,Shade5/coala,mr-karan/coala,sils1297/coala,yashtrivedi96/coala,arush0311/coala,SanketDG/coala,vinc456/coala,Balaji2198/coala,sophiavanvalkenburg/coala,JohnS-01/coala,refeed/coala,arjunsinghy96/coala,meetmangukiya/coala,arjunsinghy96/coala,abhiroyg/coala,kartikeys98/coala,impmihai/coala,incorrectusername/coala,CruiseDevice/coala,kartikeys98/coala,coala/coala,rimacone/testing2,coala-analyzer/coala,sils1297/coala,nemaniarjun/coala,yashtrivedi96/coala,Shade5/coala,sagark123/coala,NiklasMM/coala,scottbelden/coala,damngamerz/coala,yashtrivedi96/coala,Asnelchristian/coala,nemaniarjun/coala,meetmangukiya/coala,stevemontana1980/coala,netman92/coala,shreyans800755/coala,kartikeys98/coala,Shade5/coala,sophiavanvalkenburg/coala,jayvdb/coala,NiklasMM/coala,damngamerz/coala,stevemontana1980/coala,abhiroyg/coala,Nosferatul/coala,sophiavanvalkenburg/coala,JohnS-01/coala,tushar-rishav/coala,vinc456/coala