add github downloads badge

[javeme]

Change-Id: I22bd8962d014f241ba5bfb9e1a206a44400e300f

[codecov]

Codecov Report

Merging #1666 (22cd2b0) into master (b3468c7) will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1666      +/-   ##
============================================
+ Coverage     66.91%   66.95%   +0.04%     
- Complexity     7062     7067       +5     
============================================
  Files           421      421              
  Lines         34673    34673              
  Branches       4802     4802              
============================================
+ Hits          23200    23217      +17     
+ Misses         9128     9110      -18     
- Partials       2345     2346       +1     

Impacted Files	Coverage Δ
...c/main/java/com/baidu/hugegraph/task/HugeTask.java	72.30% <0.00%> (+0.30%)	⬆️
...egraph/backend/store/cassandra/CassandraStore.java	72.64% <0.00%> (+0.85%)	⬆️
.../java/com/baidu/hugegraph/auth/RolePermission.java	91.76% <0.00%> (+1.17%)	⬆️
...in/java/com/baidu/hugegraph/auth/HugeResource.java	79.22% <0.00%> (+1.29%)	⬆️
...va/com/baidu/hugegraph/auth/HugeAuthenticator.java	40.21% <0.00%> (+5.43%)	⬆️

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