[Docs] Add license header

[ckkelvinchan]

Reference: open-mmlab/mmcv#1181

Fix #493

[codecov]

Codecov Report

Merging #496 (0cc4fc4) into master (030a76b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #496   +/-   ##
=======================================
  Coverage   80.48%   80.48%           
=======================================
  Files         190      190           
  Lines       10344    10344           
  Branches     1533     1533           
=======================================
  Hits         8325     8325           
  Misses       1790     1790           
  Partials      229      229           

Flag	Coverage Δ
unittests	80.46% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmedit/__init__.py	50.00% <ø> (ø)
mmedit/apis/__init__.py	100.00% <ø> (ø)
mmedit/apis/generation_inference.py	23.07% <ø> (ø)
mmedit/apis/inpainting_inference.py	30.76% <ø> (ø)
mmedit/apis/matting_inference.py	19.51% <ø> (ø)
mmedit/apis/restoration_inference.py	16.66% <ø> (ø)
mmedit/apis/restoration_video_inference.py	16.66% <ø> (ø)
mmedit/apis/test.py	24.75% <ø> (ø)
mmedit/apis/train.py	18.00% <ø> (ø)
mmedit/core/__init__.py	100.00% <ø> (ø)
... and 176 more

---

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 030a76b...0cc4fc4. Read the comment docs.