[Solr Wiki] Trivial Update of "HowToContribute" by ErickErickson

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The "HowToContribute" page has been changed by ErickErickson:
https://wiki.apache.org/solr/HowToContribute?action=diff&rev1=105&rev2=106

Comment:
Clarified that Pull Requests should be linked to JIRAs.

  
  === Working with GitHub ===
  If you prefer you could use the [[https://github.com/apache/lucene-solr|GitHub]] mirror instead. Note that the drop-down lets you select "master" or "branch_5x" (among others).
- We accept GitHub Pull Requests (PR). To submit one, first fork the project, then make changes in a feature branch which you push to GitHub and use as the basis for the PR.
+ We accept GitHub Pull Requests (PR). To submit one, first fork the project, then make changes in a feature branch which you push to GitHub and use as the basis for the PR. PRs should be linked to a JIRA in the appropriate project here: [[http://issues.apache.org/jira/browse/SOLR|Solr Jira]] or here: [[http://issues.apache.org/jira/browse/LUCENE|Lucene Jira]]
  
  (!) '''Note about old forks:''' If you had an existing GitHub fork from before January 23rd 2016, then it will no longer list as "forked from" the official `apache/lucene-solr` project, meaning that PRs will not go to the Lucene project. Technically this is because the old `apache/lucene-solr` GitHub repo was delete and then re-added during the git switch, causing all forks to choose another "parent" project. The only fix for this is to delete your fork on the GitHub website and re-fork from the correct project.