[Docs] Refine reamde and installation

[zhouzaida]

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Motivation

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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.

[codecov]

Codecov Report

Base: 67.02% // Head: 67.02% // No change to project coverage 👍

Coverage data is based on head (ad4b958) compared to base (cb2eb57).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #2340   +/-   ##
=======================================
  Coverage   67.02%   67.02%           
=======================================
  Files         120      120           
  Lines        7737     7737           
  Branches     1059     1059           
=======================================
  Hits         5186     5186           
  Misses       2386     2386           
  Partials      165      165           

Flag	Coverage Δ
unittests	67.02% <ø> (ø)

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