Add API documentation

[rhpvorderman]

Fixes #8

Checklist

• Pull request details were added to CHANGELOG.rst

[codecov-commenter]

Codecov Report

Merging #12 into develop will decrease coverage by 0.23%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop      #12      +/-   ##
===========================================
- Coverage    66.66%   66.42%   -0.24%     
===========================================
  Files            2        2              
  Lines          141      140       -1     
===========================================
- Hits            94       93       -1     
  Misses          47       47              

Impacted Files	Coverage Δ
src/isal/igzip.py	65.44% <0.00%> (-0.26%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5cadf25...02f188e. Read the comment docs.