[Docs] Polish CONTRIBUTING for all OpenMMLab projects (#807)

* polish CONTRIBUTING for all OpenMMLab projects

* polish some description

CONTRIBUTING.md

@@ -1,4 +1,4 @@
-# Contributing to MMCV
+# Contributing to OpenMMLab
 
 All kinds of contributions are welcome, including but not limited to the following.
 
@@ -7,7 +7,7 @@ All kinds of contributions are welcome, including but not limited to the followi
 
 ## Workflow
 
-1. fork and pull the latest MMCV
+1. fork and pull the latest OpenMMLab repository
 2. checkout a new branch (do not use master branch for PRs)
 3. commit your changes
 4. create a PR
@@ -22,14 +22,16 @@ We adopt [PEP8](https://www.python.org/dev/peps/pep-0008/) as the preferred code
 
 We use the following tools for linting and formatting:
 
-- [flake8](http://flake8.pycqa.org/en/latest/): linter
-- [yapf](https://github.com/google/yapf): formatter
-- [isort](https://github.com/timothycrosley/isort): sort imports
+- [flake8](http://flake8.pycqa.org/en/latest/): A wrapper around some linter tools.
+- [yapf](https://github.com/google/yapf): A formatter for Python files.
+- [isort](https://github.com/timothycrosley/isort): A Python utility to sort imports.
+- [markdownlint](https://github.com/markdownlint/markdownlint): A linter to check markdown files and flag style issues.
+- [docformatter](https://github.com/myint/docformatter): A formatter to format docstring.
 
 Style configurations of yapf and isort can be found in [setup.cfg](./setup.cfg).
 
-We use [pre-commit hook](https://pre-commit.com/) that checks and formats for `flake8`, `yapf`, `isort`, `trailing whitespaces`,
- fixes `end-of-files`, sorts `requirments.txt` automatically on every commit.
+We use [pre-commit hook](https://pre-commit.com/) that checks and formats for `flake8`, `yapf`, `isort`, `trailing whitespaces`, `markdown files`,
+fixes `end-of-files`, `double-quoted-strings`, `python-encoding-pragma`, `mixed-line-ending`, sorts `requirments.txt` automatically on every commit.
 The config for a pre-commit hook is stored in [.pre-commit-config](./.pre-commit-config.yaml).
 
 After you clone the repository, you will need to install initialize pre-commit hook.
@@ -44,6 +46,20 @@ From the repository folder
 pre-commit install
 ```
 
+Try the following steps to install ruby when you encounter an issue on installing markdownlint
+
+```shell
+# install rvm
+curl -L https://get.rvm.io | bash -s -- --autolibs=read-fail
+[[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm"
+rvm autolibs disable
+
+# install ruby
+rvm install 2.7.1
+```
+
+Or refer to [this repo](https://github.com/innerlee/setup) and take [`zzruby.sh`](https://github.com/innerlee/setup/blob/master/zzruby.sh) according its instruction.
+
 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.