[Community] Add 'Projects/' folder, and the first example project

[gaotongxiao]

Motivation

Introducing new models and features into OpenMMLab's algorithm libraries has long been complained to be troublesome due to the rigorous requirements on code quality, which could hinder the fast iteration of SOTA models and might discourage potential contributors from sharing their latest outcome here.

Modification

This PR adds a new projects/ folder, which will be a place for some experimental models/features. Implementations inside might be not quite perfect but already fine to produce some exciting results. We hope that this PR can help us better embrace the contribution from our community. We also add the first example project to illustrate what we expect a good project to have.

[codecov]

Codecov Report

Base: 85.68% // Head: 85.76% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (640c77a) compared to base (8737675).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #1524      +/-   ##
===========================================
+ Coverage    85.68%   85.76%   +0.07%     
===========================================
  Files          158      159       +1     
  Lines         9881     9928      +47     
  Branches      1368     1377       +9     
===========================================
+ Hits          8467     8515      +48     
+ Misses        1170     1166       -4     
- Partials       244      247       +3     

Flag	Coverage Δ
unittests	85.76% <ø> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
...ocr/datasets/preparers/parsers/icdar_txt_parser.py	82.35% <0.00%> (-5.46%)	⬇️
.../models/textdet/postprocessors/db_postprocessor.py	90.41% <0.00%> (-2.74%)	⬇️
mmocr/datasets/transforms/textdet_transforms.py	88.88% <0.00%> (-2.65%)	⬇️
mmocr/datasets/preparers/data_converter.py	20.61% <0.00%> (-0.10%)	⬇️
mmocr/datasets/preparers/parsers/base.py	72.00% <0.00%> (ø)
mmocr/datasets/preparers/data_obtainer.py	20.73% <0.00%> (ø)
mmocr/datasets/preparers/parsers/svt_parser.py	100.00% <0.00%> (ø)
mmocr/utils/bbox_utils.py	61.84% <0.00%> (+1.60%)	⬆️
...cr/models/textdet/module_losses/pan_module_loss.py	96.79% <0.00%> (+7.69%)	⬆️
...r/models/kie/postprocessors/sdmgr_postprocessor.py	94.11% <0.00%> (+8.82%)	⬆️

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