add dependabot config

[Robert-Steiner]

adds and enables dependabot for:

• xaynet workspace (checks daily whether new versions of the dependencies are available)
• python bindings (weekly checks)
• dependencies of the keras example (weekly checks)
• github actions (weekly checks)
• disable binary auto discovery for examples (Cargo lib with name 'examples` incorrectly classified as binary dependabot/dependabot-core#1156)
• updated README

[codecov]

Codecov Report

Merging #649 (02f25cb) into master (78181bc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #649   +/-   ##
=======================================
  Coverage   63.77%   63.77%           
=======================================
  Files          87       87           
  Lines        4190     4190           
=======================================
  Hits         2672     2672           
  Misses       1518     1518           

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[Robert-Steiner]

I forgot to disable the binary auto discovery of cargo. Dependabot complains that the binary target name examples is forbidden. I added links of similar issues to the setting. During a test, I noticed the command in the readme file wasn't working, so I updated the readme as well.