Add a note about fixing PANTS_VERSION mismatch.
Add a note about fixing PANTS_VERSION mismatch.

This can easily happen when running pants from source against a repo
that hasn't updated pants in a while.

Testing Done:
Rendered here:
  http://pantsbuild.github.io/staging/jsirois/dev-notes/pants-from-source/howto_develop.html

CI went green here:
  http://jenkins.pantsbuild.org/job/pantsbuild/job/pants/branch/PR-3441/1/

Bugs closed: 3417, 3441

Reviewed at https://rbcommons.com/s/twitter/r/3887/

twitter/pants,gmalmquist/pants,ericzundel/pants,pantsbuild/pants,manasapte/pants,foursquare/pants,gmalmquist/pants,baroquebobcat/pants,wisechengyi/pants,wisechengyi/pants,UnrememberMe/pants,jsirois/pants,UnrememberMe/pants,pantsbuild/pants,UnrememberMe/pants,tdyas/pants,pombredanne/pants,mateor/pants,manasapte/pants,gmalmquist/pants,twitter/pants,fkorotkov/pants,fkorotkov/pants,ericzundel/pants,ity/pants,kwlzn/pants,gmalmquist/pants,tdyas/pants,lahosken/pants,cevaris/pants,baroquebobcat/pants,ity/pants,15Dkatz/pants,landism/pants,peiyuwang/pants,peiyuwang/pants,fkorotkov/pants,lahosken/pants,mateor/pants,tdyas/pants,peiyuwang/pants,ity/pants,landism/pants,UnrememberMe/pants,pombredanne/pants,foursquare/pants,pombredanne/pants,baroquebobcat/pants,pombredanne/pants,peiyuwang/pants,lahosken/pants,pantsbuild/pants,wisechengyi/pants,fkorotkov/pants,benjyw/pants,wisechengyi/pants,tdyas/pants,gmalmquist/pants,twitter/pants,foursquare/pants,foursquare/pants,pantsbuild/pants,pantsbuild/pants,ericzundel/pants,benjyw/pants,tdyas/pants,mateor/pants,baroquebobcat/pants,twitter/pants,wisechengyi/pants,cevaris/pants,landism/pants,manasapte/pants,mateor/pants,cevaris/pants,benjyw/pants,twitter/pants,benjyw/pants,manasapte/pants,manasapte/pants,wisechengyi/pants,pombredanne/pants,fkorotkov/pants,ericzundel/pants,kwlzn/pants,landism/pants,foursquare/pants,twitter/pants,gmalmquist/pants,pantsbuild/pants,tdyas/pants,pantsbuild/pants,pombredanne/pants,manasapte/pants,mateor/pants,ity/pants,cevaris/pants,ity/pants,ericzundel/pants,baroquebobcat/pants,UnrememberMe/pants,baroquebobcat/pants,cevaris/pants,kwlzn/pants,15Dkatz/pants,lahosken/pants,peiyuwang/pants,benjyw/pants,baroquebobcat/pants,15Dkatz/pants,lahosken/pants,UnrememberMe/pants,foursquare/pants,wisechengyi/pants,kwlzn/pants,lahosken/pants,lahosken/pants,landism/pants,pombredanne/pants,twitter/pants,peiyuwang/pants,landism/pants,foursquare/pants,lahosken/pants,15Dkatz/pants,15Dkatz/pants,UnrememberMe/pants,15Dkatz/pants,15Dkatz/pants,foursquare/pants,ericzundel/pants,tdyas/pants,kwlzn/pants,benjyw/pants,twitter/pants,mateor/pants,baroquebobcat/pants,benjyw/pants,ity/pants,fkorotkov/pants,fkorotkov/pants,peiyuwang/pants,wisechengyi/pants,ity/pants,tdyas/pants,cevaris/pants,landism/pants,mateor/pants,jsirois/pants,fkorotkov/pants,kwlzn/pants,wisechengyi/pants,ericzundel/pants,cevaris/pants,UnrememberMe/pants,tdyas/pants,landism/pants,baroquebobcat/pants,kwlzn/pants,ericzundel/pants,foursquare/pants,manasapte/pants,UnrememberMe/pants,jsirois/pants,mateor/pants,gmalmquist/pants,15Dkatz/pants,twitter/pants,peiyuwang/pants