Create CONTRIBUTING.md (#102)

CONTRIBUTING.md

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+# Contributing to mmcv
+
+All kinds of contributions are welcome, including but not limited to the following.
+
+- Fixes (typo, bugs)
+- New features and components
+
+## Workflow
+
+1. fork and pull the latest mmcv
+2. checkout a new branch (do not use master branch for PRs)
+3. commit your changes
+4. create a PR
+
+Note: If you plan to add some new features that involve large changes, it is encouraged to open an issue for discussion first.
+
+## Code style
+
+### Python
+We adopt [PEP8](https://www.python.org/dev/peps/pep-0008/) as the preferred code style.
+
+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
+
+Style configurations of yapf and isort can be found in [.style.yapf](.style.yapf) and [.isort.cfg](.isort.cfg).
+
+>Before you create a PR, make sure that your code lints and is formatted by yapf.
+
+### C++ and CUDA
+We follow the [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html).