git remote add upstream useing https (#690)

shonge · weekface · commit 4eed6a328d01 · 2019-07-29T14:45:55.000+08:00
(cherry picked from commit 0333f97)
diff --git a/docs/CONTRIBUTING.md b/docs/CONTRIBUTING.md
@@ -58,7 +58,7 @@ Set your clone to track upstream repository.
 
 ```sh
 $ cd $working_dir/tidb-operator
-$ git remote add upstream git@github.com:pingcap/tidb-operator.git
+$ git remote add upstream https://github.com/pingcap/tidb-operator
 ```
 
 Since you don't have write access to the upstream repository, you need to disable pushing to upstream master:
