[Solr Wiki] Update of "HowToContribute" by TomasFernandezLobbe

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The "HowToContribute" page has been changed by TomasFernandezLobbe:
https://wiki.apache.org/solr/HowToContribute?action=diff&rev1=106&rev2=107

  
  === 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. 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]]
+ 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 issue 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]]. To link your PR to the Jira issue simply include the Jira issue id in the title of the PR (first create the Jira issue if one doesn't exist). 
  
  (!) '''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.