add CONTRIBUTING.md, fix whats_new STYLE

[sappelhoff]

I turned the instructions I wrote for #9 (comment) into this CONTRIBUTING.md file.

It may be helpful for other contributors in the future.

Feel free to comment and improve, else I'll merge this tomorrow.

[codecov-io]

Codecov Report

Merging #22 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   97.20%   97.20%           
=======================================
  Files           6        6           
  Lines         643      644    +1     
=======================================
+ Hits          625      626    +1     
  Misses         18       18           

Impacted Files	Coverage Δ
pyprep/find_noisy_channels.py	95.49% <100.00%> (+0.02%)	⬆️
pyprep/reference.py	97.19% <100.00%> (ø)

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