Release v0.15.0 - Githubissues

Update README, etc. See delta in #123 for details.

[x] update VERSION.txt version to v0.15.0
[x] update library.properties version to 0.15.0
[x] update README.md version for badges (line 7)
[x] update README.md release history (towards end).
[x] update kVersion in CatenaBase: change here: https://github.com/mcci-catena/Catena-Arduino-Platform/blob/8f3995d3c6f6bd5207ec7f5678bc122bd04cad83/src/CatenaBase.h#L94
[x] generate pull request
[x] merge into master.
[x] tag and release.

mcci-catena / Catena-Arduino-Platform