[Docs] Add solution for markdown-lint installation issue (open-mmlab#497

)

* add solution for markdown-lint issue

* add setup

* simplify ruby install

.github/CONTRIBUTING.md

@@ -49,6 +49,20 @@ From the repository folder
 pre-commit install
 ```
 
+If you are facing an issue when installing markdown lint, you may install ruby for markdown lint by
+referring to [this repo](https://github.com/innerlee/setup) by following the usage and taking [`zzruby.sh`](https://github.com/innerlee/setup/blob/master/zzruby.sh)
+
+or by the following steps
+
+```shell
+# install rvm
+curl -L https://get.rvm.io | bash -s -- --autolibs=read-fail
+rvm autolibs disable
+
+# install ruby
+rvm install 2.7.1
+```
+
 After this on every commit check code linters and formatter will be enforced.
 
 >Before you create a PR, make sure that your code lints and is formatted by yapf.