[Solr Wiki] Update of "HowToContribute" by JamieJackson

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The "HowToContribute" page has been changed by JamieJackson:
https://wiki.apache.org/solr/HowToContribute?action=diff&rev1=108&rev2=109

Comment:
+ note about GitHub mirror (earlier on); + git checkout command (which had been referenced, but absent)

  
  Get the source code on your local drive using  [[http://lucene.apache.org/solr/resources.html#solr-version-control]].
  
- To check out code from GIT
+ To check out code from Git. (The repositories listed in the commands, below, use the Apache Git repository; however, a GitHub mirror is also available. See ''[[#Working_with_GitHub|Working with GitHub]]''.)
  
  {{{
- for non-committers:
+ # clone repository - for non-committers (read-only):
  git clone http://git-wip-us.apache.org/repos/asf/lucene-solr.git
  
- for committers
+ # clone repository - for committers (read/write):
  git clone https://git-wip-us.apache.org/repos/asf/lucene-solr.git
+ 
+ # check out desired branch:
+ git checkout <branch>
  }}}
  The <branch> part of the command above needs to be replaced by something concrete - the "code line" you want to get.  Examples, and how to interpret what they mean: