Added 1.20.12 release notes.
Merge pull request #210 from sinzui/1.20.12-release-notes

Added 1.20.12 release notes.
Added 1.20.13 release notes.
Merge pull request #211 from sinzui/1.20.13-release-notes

Added 1.20.13 release notes.
minor formatting changes
add empty default string to example.
Add simple note about policy reasoning.
Merge pull request #213 from dpb1/add-empty-default

Add empty default parameter to charm docs