Polish readme

[zhouzaida]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Polish README

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[hellock]

Another option: just rename the path from 1.8.0 to 1.8.

[codecov]

Codecov Report

Merging #1320 (0dfb91e) into master (6434d0d) will not change coverage.
The diff coverage is n/a.

❗ Current head 0dfb91e differs from pull request most recent head 44b3fb8. Consider uploading reports for the commit 44b3fb8 to get more accurate results

@@           Coverage Diff           @@
##           master    #1320   +/-   ##
=======================================
  Coverage   69.14%   69.14%           
=======================================
  Files         162      162           
  Lines       10746    10746           
  Branches     1978     1978           
=======================================
  Hits         7430     7430           
  Misses       2927     2927           
  Partials      389      389           

Flag	Coverage Δ
unittests	69.14% <ø> (ø)

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