doc: add code style config file for IDEA

[imbajin]

And also update contributing doc

[codecov]

Codecov Report

Merging #1060 into master will increase coverage by 0.19%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1060      +/-   ##
============================================
+ Coverage     68.97%   69.17%   +0.19%     
- Complexity     5411     5437      +26     
============================================
  Files           328      328              
  Lines         26400    26400              
  Branches       3773     3773              
============================================
+ Hits          18209    18261      +52     
+ Misses         6402     6356      -46     
+ Partials       1789     1783       -6     

Impacted Files	Coverage Δ	Complexity Δ
...du/hugegraph/backend/tx/GraphIndexTransaction.java	79.85% <0.00%> (+0.11%)	192.00% <0.00%> (ø%)
...graph/backend/store/cassandra/CassandraTables.java	88.14% <0.00%> (+0.26%)	3.00% <0.00%> (ø%)
...om/baidu/hugegraph/task/StandardTaskScheduler.java	85.61% <0.00%> (+0.35%)	67.00% <0.00%> (ø%)
...egraph/backend/store/cassandra/CassandraStore.java	69.01% <0.00%> (+0.35%)	53.00% <0.00%> (+1.00%)
.../baidu/hugegraph/backend/query/ConditionQuery.java	86.20% <0.00%> (+0.43%)	101.00% <0.00%> (+1.00%)
.../hugegraph/backend/serializer/TableSerializer.java	87.90% <0.00%> (+0.53%)	61.00% <0.00%> (+1.00%)
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	80.11% <0.00%> (+1.11%)	297.00% <0.00%> (+5.00%)
...java/com/baidu/hugegraph/type/define/DataType.java	86.13% <0.00%> (+1.98%)	52.00% <0.00%> (+2.00%)
...du/hugegraph/traversal/optimize/TraversalUtil.java	59.17% <0.00%> (+2.52%)	118.00% <0.00%> (+4.00%)
...egraph/backend/store/cassandra/CassandraTable.java	80.14% <0.00%> (+4.25%)	84.00% <0.00%> (+4.00%)
... and 4 more

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