Update notes for submitted changes
* reftable: reserve standard PackExt
* reftable: resolve symbolic references
* reftable: support threshold based compaction
* reftable: compact merged tables
* reftable: merge-join reftables
* reftable: lookup by ObjectId unit tests
* reftable: reflog unit tests
* reftable: namespace unit tests
* reftable: bulk operation unit tests
* reftable: basic functionality unit tests
* reftable: debug tools
* reftable: scan and lookup reftable files
* reftable: create and write reftable files
* reftable: file format documentation