docs: update README doc

[Phoenix500526]

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov[bot]]

Codecov Report

Attention: 634 lines in your changes are missing coverage. Please review.

Comparison is base (fa656ad) 64.40% compared to head (318f966) 79.76%. Report is 112 commits behind head on master.

Files	Patch %	Lines
curp/src/client/unary.rs	86.28%	48 Missing :warning:
curp/src/server/curp_node.rs	82.37%	46 Missing :warning:
curp/src/server/raw_curp/mod.rs	83.15%	46 Missing :warning:
xline/src/storage/alarm_store.rs	67.88%	44 Missing :warning:
curp/src/rpc/connect.rs	61.81%	42 Missing :warning:
utils/src/parser.rs	84.79%	33 Missing :warning:
xline/src/storage/kv_store.rs	23.07%	30 Missing :warning:
xline/src/server/maintenance.rs	63.29%	29 Missing :warning:
curp/src/client/tests.rs	94.25%	24 Missing :warning:
curp/src/client/state.rs	88.66%	23 Missing :warning:
... and 30 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #634 +/- ## =========================================== + Coverage 64.40% 79.76% +15.36% =========================================== Files 105 153 +48 Lines 18802 22920 +4118 Branches 18802 0 -18802 =========================================== + Hits 12109 18283 +6174 + Misses 5955 4637 -1318 + Partials 738 0 -738 ```

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