chore: add logo pic for README

[imbajin]

also make subtitle clearly

the preview:

[codecov]

Codecov Report

Merging #1731 (c0f582b) into master (e9cef74) will increase coverage by 0.61%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1731      +/-   ##
============================================
+ Coverage     66.46%   67.07%   +0.61%     
- Complexity      684      972     +288     
============================================
  Files           443      443              
  Lines         37678    37678              
  Branches       5361     5361              
============================================
+ Hits          25042    25274     +232     
+ Misses        10035     9800     -235     
- Partials       2601     2604       +3     

Impacted Files	Coverage Δ
...va/com/baidu/hugegraph/util/collection/IntSet.java	74.04% <0.00%> (-1.28%)	⬇️
...va/com/baidu/hugegraph/util/collection/IntMap.java	72.40% <0.00%> (-0.40%)	⬇️
...om/baidu/hugegraph/task/StandardTaskScheduler.java	75.85% <0.00%> (+0.24%)	⬆️
...aidu/hugegraph/backend/store/mysql/MysqlTable.java	81.59% <0.00%> (+0.54%)	⬆️
...u/hugegraph/backend/store/mysql/MysqlSessions.java	69.06% <0.00%> (+2.54%)	⬆️
...aidu/hugegraph/backend/tx/AbstractTransaction.java	75.00% <0.00%> (+2.97%)	⬆️
...aidu/hugegraph/backend/store/mysql/MysqlStore.java	83.75% <0.00%> (+3.04%)	⬆️
...h/backend/store/postgresql/PostgresqlSessions.java	77.50% <0.00%> (+77.50%)	⬆️
...backend/store/postgresql/PostgresqlSerializer.java	78.57% <0.00%> (+78.57%)	⬆️
...aph/backend/store/postgresql/PostgresqlTables.java	87.95% <0.00%> (+87.95%)	⬆️
... and 3 more

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