Add missing documentation

[Mr-Geekman]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

Before submitting (checklist)

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contribution guide?
• Did you check the code style? make lint (poetry install -E style).
• Did you make sure to update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you check that your code passes the unit tests pytest tests/ ?
• Did you add your new functionality to the docs?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Related Issue

#54.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Closing issues

Closes #54.

[codecov-commenter]

Codecov Report

Merging #55 (34fca22) into master (26d88da) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   90.33%   90.33%           
=======================================
  Files          45       45           
  Lines        1976     1976           
=======================================
  Hits         1785     1785           
  Misses        191      191           

Impacted Files	Coverage Δ
etna/models/catboost.py	100.00% <ø> (ø)
etna/models/sklearn.py	100.00% <ø> (ø)

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