Various repo fixes

[FelixWolf]

These fixes include:

1. Dark theme compatible markdown that uses black on white text if light theme(original), or the white on black text if dark theme(addition). The addition is version I modified to match that of the forum's dark theme:
   Old:
   
   New:
   

Issues: I have no idea if bitbucket supports HTML markdown. By markdown standard, it should, but would be worth double checking if bitbucket is still intended to be pushed to.

2. LGPL-licence.txt renamed to LGPL-license.txt. Licence is the verb to give a license, License is the noun of a document. (Note, github sees this as a file move for some reason)
3. Added copy of LGPL-license.txt as LICENSE.txt so that github recognizes it as a license and displays it as such on the sidebar. (Note, github sees this as a new file for some reason)

[bennettgoble]

Hey, @FelixWolf. Thanks for the PR. Could you pull in the latest changes from master in order to incorporate our new CLA bot?

[bennettgoble]

Also, could you use LICENSE instead of LICENSE.txt? This would match the name being used in our other public repositories such as autobuild and leap. :)

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[FelixWolf]

I have read the CLA Document and I hereby sign the CLA

[bennettgoble]

LGTM. The README looks much better in dark mode now.