remove 'Changes to Backwards Compatibility Policy' from lucene/CHANGES.txt [LUCENE-4943]

CHANGES.txt is useful to summarize the changes in a release.

However its expected that a lot of changes will impact the APIs, this currently hurts the quality of CHANGES.txt because it leads to a significant portion of changes (whether they be bugs, features, whatever) being grouped under this one title.

It also leads to descriptions of CHANGES being unnecessarily verbose.

I think it makes CHANGES confusing and overwhelming, and it would be better to have a simpler 'upgrading' section with practical information on what you actually need to do (like Solr's CHANGES.txt).

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Migrated from [LUCENE-4943](https://issues.apache.org/jira/browse/LUCENE-4943) by Robert Muir (@rmuir), updated May 09 2016

Start with lucene/CHANGES.txt and compare its current structure with Solr's CHANGES.txt. Identify how the “Changes to Backwards Compatibility Policy” material is currently grouped, then determine the practical upgrading information that should replace it. Done means the release notes are simpler and no longer use that heading, but the issue does not specify the exact replacement content.

Written by the indexing model from the issue text.