Release 0.19.0 - Githubissues

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

[x] ~update VERSION.txt version to v0.19.0~
[x] update library.properties version to 0.19.0
[x] update CATENA_ARDUINO_PLATFORM_VERSION in CatenaBase: change here: https://github.com/mcci-catena/Catena-Arduino-Platform/blob/1de835c93c4f76908e6fba119732fe45d58db7f6/src/CatenaBase.h#L58
[x] update README.md version for badges (line 7)
[x] update README.md release history (towards end).
[ ] generate pull request
[ ] merge into master.
[ ] tag and release.

mcci-catena / Catena-Arduino-Platform