Add CONTRIBUTING.md (#152)

* add CONTRIBUTING

* Rename CONTRIBUTING.MD to CONTRIBUTING.md

* add contribute to README

CONTRIBUTING.md

@@ -0,0 +1,67 @@
+# Contributing to OpenZeppelin Nile
+
+We really appreciate and value contributions to OpenZeppelin Nile. Please take 5' to review the items listed below to make sure that your contributions are merged as soon as possible.
+
+## Contribution guidelines
+
+Before starting development, please [create an issue](https://github.com/OpenZeppelin/nile/issues/new) to open the discussion, validate that the PR is wanted, and coordinate overall implementation details.
+
+And make sure to always include tests and documentation for the new developments.
+
+## Creating Pull Requests (PRs)
+
+As a contributor, you are expected to fork this repository, work on your own fork and then submit pull requests. The pull requests will be reviewed and eventually merged into the main repo. See ["Fork-a-Repo"](https://help.github.com/articles/fork-a-repo/) for how this works.
+
+## A typical workflow
+
+1. Make sure your fork is up to date with the main repository:
+
+    ```sh
+    cd nile
+    git remote add upstream https://github.com/OpenZeppelin/nile.git
+    git fetch upstream
+    git pull --rebase upstream main
+    ```
+
+    > NOTE: The directory `nile` represents your fork's local copy.
+
+2. Branch out from `main` into `fix/some-bug-#123`:
+
+    (Postfixing #123 will associate your PR with the issue #123 and make everyone's life easier =D)
+
+    ```sh
+    git checkout -b fix/some-bug-#123
+    ```
+
+3. Make your changes, add your files, commit, and push to your fork.
+
+    ```sh
+    git add some_file.py
+    git commit "Fix some bug #123"
+    git push origin fix/some-bug-#123
+    ```
+
+4. Run tests, linter, etc. This can be done by running local continuous integration and make sure it passes.
+
+    ```bash
+    tox
+    tox -e format
+    tox -e lint
+    ```
+
+5. Go to [github.com/OpenZeppelin/nile](https://github.com/OpenZeppelin/nile) in your web browser and issue a new pull request.
+    Begin the body of the PR with "Fixes #123" or "Resolves #123" to link the PR to the issue that it is resolving.
+    *IMPORTANT* Read the PR template very carefully and make sure to follow all the instructions. These instructions
+    refer to some very important conditions that your PR must meet in order to be accepted, such as making sure that all PR checks pass.
+
+6. Maintainers will review your code and possibly ask for changes before your code is pulled in to the main repository. We'll check that all tests pass, review the coding style, and check for general code correctness. If everything is OK, we'll merge your pull request and your code will be part of OpenZeppelin Nile.
+
+    *IMPORTANT* Please pay attention to the maintainer's feedback, since its a necessary step to keep up with the standards OpenZeppelin Nile attains to.
+
+## All set
+
+If you have any questions, feel free to post them to github.com/OpenZeppelin/nile/issues.
+
+Finally, if you're looking to collaborate and want to find easy tasks to start, look at the issues we marked as ["Good first issue"](https://github.com/OpenZeppelin/nile/labels/good%20first%20issue).
+
+Thanks for your time and code!

README.md

@@ -370,6 +370,10 @@ How to decide if I want to use a plugin or not? Just install / uninstall the plu
 
 Finally, after the desired plugin is installed, it will also be automatically available through the `nre`. The plugin developer should be aware of this and design the interface accordingly.
 
+## Contribute
+
+OpenZeppelin Nile exists thanks to its contributors. There are many ways you can participate and help build high quality software. Check out the [contribution](CONTRIBUTING.md) guide!
+
 ## Hacking on Nile
 
 Nile uses tox to manage development tasks, you can get a list of