Add 0.5.4 release notes.

version 0.5.3

version 0.5.2

version 0.5.2rc2

Version 0.5.2rc1

Version 0.5.1

Version 0.5.0

Move to experimental types in the docs

Merge pull request #154 from cpcloud/typevar-docs

Clarify symbolic type syntax

Make sure our version is not unknown