[Docs] Revise install and reorg README

[MeowZheng]

related pr open-mmlab/mmdetection#7897

[codecov]

Codecov Report

Merging #139 (70c57d8) into master (5f5fed8) will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   68.58%   68.72%   +0.13%     
==========================================
  Files          92       92              
  Lines        5148     5148              
  Branches      733      733              
==========================================
+ Hits         3531     3538       +7     
+ Misses       1502     1495       -7     
  Partials      115      115              

Flag	Coverage Δ
unittests	68.72% <ø> (+0.13%)	⬆️

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

Impacted Files	Coverage Δ
mmflow/datasets/pipelines/transforms.py	78.66% <0.00%> (+1.16%)	⬆️

---

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 5f5fed8...70c57d8. Read the comment docs.