CONTRIBUTING.md: add how to create a GitHub token

[javeme]

No description provided.

[codecov]

Codecov Report

Merging #1820 (9e7c702) into master (73a1f1f) will increase coverage by 4.32%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1820      +/-   ##
============================================
+ Coverage     66.56%   70.89%   +4.32%     
- Complexity      978      980       +2     
============================================
  Files           446      446              
  Lines         37902    37902              
  Branches       5398     5398              
============================================
+ Hits          25230    26870    +1640     
+ Misses         9967     8301    -1666     
- Partials       2705     2731      +26     

Impacted Files	Coverage Δ
...du/hugegraph/backend/tx/GraphIndexTransaction.java	83.82% <0.00%> (+0.21%)	⬆️
...du/hugegraph/traversal/optimize/TraversalUtil.java	62.87% <0.00%> (+0.21%)	⬆️
.../baidu/hugegraph/backend/query/ConditionQuery.java	85.64% <0.00%> (+0.24%)	⬆️
...du/hugegraph/schema/builder/IndexLabelBuilder.java	88.69% <0.00%> (+0.28%)	⬆️
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	80.08% <0.00%> (+0.30%)	⬆️
.../com/baidu/hugegraph/auth/StandardAuthManager.java	92.81% <0.00%> (+0.32%)	⬆️
...idu/hugegraph/schema/builder/EdgeLabelBuilder.java	83.62% <0.00%> (+0.34%)	⬆️
...u/hugegraph/schema/builder/VertexLabelBuilder.java	87.54% <0.00%> (+0.37%)	⬆️
.../java/com/baidu/hugegraph/backend/query/Query.java	86.11% <0.00%> (+0.39%)	⬆️
...hugegraph/backend/query/ConditionQueryFlatten.java	76.15% <0.00%> (+0.41%)	⬆️
... and 111 more

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[imbajin]

seems it's not necessary for normal user/dev to use this?

[javeme]

mainly for new contributors

[imbajin]

todo:

• we should let dev use ssh to push code later (not recommend for http way)