Add warning section about look-ahead bias

[Mr-Geekman]

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

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• 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

Look #390

Related Issue

#390

Closing issues

Close #390

[Mr-Geekman]

That is how it looks in the docs:

[codecov-commenter]

Codecov Report

Merging #464 (6a6cf6c) into master (70c5fac) will decrease coverage by 34.33%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #464       +/-   ##
===========================================
- Coverage   87.98%   53.64%   -34.34%     
===========================================
  Files         115      115               
  Lines        5441     5441               
===========================================
- Hits         4787     2919     -1868     
- Misses        654     2522     +1868     

Impacted Files	Coverage Δ
etna/transforms/decomposition/binseg.py	100.00% <ø> (ø)
...na/transforms/decomposition/change_points_trend.py	40.18% <ø> (-58.88%)	⬇️
etna/transforms/decomposition/detrend.py	41.66% <ø> (-56.67%)	⬇️
etna/transforms/decomposition/stl.py	58.57% <ø> (-35.72%)	⬇️
etna/transforms/decomposition/trend.py	45.94% <ø> (-54.06%)	⬇️
etna/transforms/math/power.py	100.00% <ø> (ø)
etna/transforms/math/scalers.py	100.00% <ø> (ø)
etna/transforms/missing_values/imputation.py	34.48% <ø> (-65.52%)	⬇️
etna/transforms/missing_values/resample.py	24.13% <ø> (-68.97%)	⬇️
etna/transforms/outliers/point_outliers.py	92.68% <ø> (-7.32%)	⬇️
... and 76 more

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[martins0n]

👍