[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=107&rev2=108

Comment:
Updated that current master is eventual 7.0 etc.

  }}}
  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:
  
-  * master: Working towards the eventual 6.0 release. This is the main center for development, it's not really a branch.  Releases are never made from master, they are only made from the stable development branch.
+  * master: Working towards the eventual 7.0 release. This is the main center for development, it's not really a branch.  Releases are never made from master, they are only made from the stable development branch.
-  * branch_5x: The current stable development branch for the next stable 5.x version.
+  * branch_6x: The current stable development branch for the next stable 6.x version.
+  * releases/lucene-solr/5.5.3: When a new version is fully released, the lucene_solr_x_x branch is copied to a tag which represents the source code for that specific release.
   * releases/lucene-solr/4.10.3: When a new version is fully released, the lucene_solr_x_x branch is copied to a tag which represents the source code for that specific release.
   * history/branches/lucene-solr/lucene_solr_4_10: When branch_4x was removed and 4.x moved to maintenance mode, all 4.x development moved to this branch.  There is never a guarantee that a point release will ever be made after the x.x.0 version is released.
   * history/branches/lucene-solr/lucene_solr_3_6: When branch_3x was removed and 3.x moved to maintenance mode, all 3.x development moved to this branch.