[Lucene-java Wiki] Update of "ReleaseNote50" by AnshumGupta

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The "ReleaseNote50" page has been changed by AnshumGupta:
https://wiki.apache.org/lucene-java/ReleaseNote50?action=diff&rev1=3&rev2=4

Comment:
Added a note about 3.x index back-compat break and what to do about it. Also added a link to Changes.html,

   * The default codec has an option to control BEST_SPEED or BEST_COMPRESSION for stored fields.
   * Stored fields are merged more efficiently, especially when upgrading from previous versions or using SortingMergePolicy
  
+ NOTE: Lucene 5 no longer supports the Lucene 3.x index format. Opening indexes will result in `IndexFormatTooOldException`. It is recommended to either reindex all your data, or upgrade the old indexes with the `IndexUpgrader` tool of latest Lucene 4 version (4.10.x). Those indexes can then be read (see next section) with Lucene 5.
+ 
  To read more about the changes, also see: http://blog.mikemccandless.com/2014/11/apache-lucene-500-is-coming.html
  
- Please read CHANGES.txt and MIGRATE.txt for a full list of new features and notes on upgrading.
+ Please read CHANGES.txt (https://lucene.apache.org/core/5_0_0/changes/Changes.html) and MIGRATE.txt for a full list of new features and notes on upgrading.
  
  Please report any feedback to the mailing lists (http://lucene.apache.org/core/discussion.html)
  }}}