Add release notes excerpt to README.md
* Renames current release notes document to release-notes.md to be copied to release-notes-1.6.0.md when no longer current
* Adds link to doc/release-notes.md to README.md
* Adds highlights excerpt from release-notes.md to README.md
Merge pull request #82 from Cheeseness/release-notes-fixes

Add release notes excerpt to README.md