Add CONTRIBUTING.md file

[juhoinkinen]

Adding this information hopefully helps newcomers when they wish to participate in Annif development.

Some parts are borrowed from Annif wiki and some from the corresponding file of Skosmos.

On next release, the parts taken here from Annif wiki should be deleted from there to avoid duplication (Development flow, branches and tags and Creating a new backend).

[codecov]

Codecov Report

Base: 99.58% // Head: 99.54% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (74ce87b) compared to base (63af34c).
Patch has no changes to coverable lines.

❗ Current head 74ce87b differs from pull request most recent head 45070c8. Consider uploading reports for the commit 45070c8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #643      +/-   ##
==========================================
- Coverage   99.58%   99.54%   -0.04%     
==========================================
  Files          87       87              
  Lines        5972     5968       -4     
==========================================
- Hits         5947     5941       -6     
- Misses         25       27       +2     

Impacted Files	Coverage Δ
annif/transform/__init__.py	93.75% <0.00%> (-6.25%)	⬇️
annif/transform/langfilter.py	100.00% <0.00%> (ø)
tests/test_transform_langfilter.py	100.00% <0.00%> (ø)

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[osma]

Here are some more ideas for what to include in a CONTRIBUTING.md file.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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