Polish release notes

[wangkuiyi]

Fixes #1955

I corrected some English errors in the released feature list and added a summarization.

Also, I had to run pre-commit autoupdate on my iMac, which changed pre-commit-config.yaml, in order to fix the following errors with pre-commit:

yi@WangYis-iMac:~/work/paddle (polish-release-note)*$ git commit -a -m 'Polish release notes'
git push -^[[24~u [WARNING] https://github.com/Lucas-C/pre-commit-hooks.git uses legacy hooks.yaml to provide hooks.
In newer versions, this file is called .pre-commit-hooks.yaml
This will work in this version of pre-commit but will be removed at a later time.
If `pre-commit autoupdate` does not silence this warning consider making an issue / pull request.
[WARNING] https://github.com/reyoung/mirrors-yapf.git uses legacy hooks.yaml to provide hooks.
In newer versions, this file is called .pre-commit-hooks.yaml
This will work in this version of pre-commit but will be removed at a later time.
If `pre-commit autoupdate` does not silence this warning consider making an issue / pull request.
[WARNING] https://github.com/pre-commit/pre-commit-hooks uses legacy hooks.yaml to provide hooks.
In newer versions, this file is called .pre-commit-hooks.yaml
This will work in this version of pre-commit but will be removed at a later time.
If `pre-commit autoupdate` does not silence this warning consider making an issue / pull request.
[WARNING] https://github.com/PaddlePaddle/clang-format-pre-commit-hook.git uses legacy hooks.yaml to provide hooks.
In newer versions, this file is called .pre-commit-hooks.yaml
This will work in this version of pre-commit but will be removed at a later time.
If `pre-commit autoupdate` does not silence this warning consider making an issue / pull request.
CRLF end-lines remover...............................(no files to check)Skipped
yapf.................................................(no files to check)Skipped
Check for added large files..............................................Failed
hookid: check-added-large-files

dyld: Library not loaded: @executable_path/../.Python
  Referenced from: /Users/yi/.pre-commit/repofz9Kw1/py_env-default/bin/python2.7
  Reason: image not found

Check for merge conflicts................................................Failed
hookid: check-merge-conflict

dyld: Library not loaded: @executable_path/../.Python
  Referenced from: /Users/yi/.pre-commit/repofz9Kw1/py_env-default/bin/python2.7
  Reason: image not found

Check for broken symlinks................................................Failed
hookid: check-symlinks

dyld: Library not loaded: @executable_path/../.Python
  Referenced from: /Users/yi/.pre-commit/repofz9Kw1/py_env-default/bin/python2.7
  Reason: image not found

Detect Private Key...................................(no files to check)Skipped
Fix End of Files.........................................................Failed
hookid: end-of-file-fixer

dyld: Library not loaded: @executable_path/../.Python
  Referenced from: /Users/yi/.pre-commit/repofz9Kw1/py_env-default/bin/python2.7
  Reason: image not found

clang-formater.......................................(no files to check)Skipped