doc: update readme #161

[hayleyling]

1. Correct some minor errors in the readme file.
2. Add icons to make it more readable.

[codecov]

Codecov Report

Merging #162 (0abafff) into master (21c30dd) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #162   +/-   ##
=====================================
- Coverage      69%    68%   -1%     
=====================================
  Files          51     51           
  Lines        4601   4601           
=====================================
- Hits         3175   3170    -5     
- Misses       1173   1177    +4     
- Partials      253    254    +1     

Impacted Files	Coverage Δ
src/raft/candidate.go	82% <0%> (-3%)	⬇️

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