Add CONTRIBUTING.md

CONTRIBUTING.md

+# How to contribute to seqeval?
+
+1. Fork the [repository](https://github.com/chakki-works/seqeval) by clicking on the 'Fork' button on the repository's page. This creates a copy of the code under your GitHub user account.
+
+2. Clone your fork to your local disk, and add the base repository as a remote:
+
+	```bash
+	git clone git@github.com:<your Github handle>/seqeval.git
+	cd seqeval
+	git remote add upstream https://github.com/chakki-works/seqeval.git
+	```
+
+3. Create a new branch to hold your development changes:
+
+	```bash
+	git checkout -b a-descriptive-name-for-my-changes
+	```
+
+	**do not** work on the `master` branch.
+
+4. Set up a development environment by running the following command in a virtual environment:
+
+	```bash
+	pipenv install --dev
+	```
+
+5. Develop the features on your branch.
+
+6. Format your code. Run flake8 and isort so that your newly added files look nice with the following command:
+
+	```bash
+	pipenv run flake8
+	pipenv run isort
+	```
+
+7. Once you're happy with your script file, add your changes and make a commit to record your changes locally:
+
+	```bash
+	git add seqeval/<your_script_name>
+	git commit
+	```
+
+	It is a good idea to sync your copy of the code with the original
+	repository regularly. This way you can quickly account for changes:
+
+	```bash
+	git fetch upstream
+	git rebase upstream/master
+    ```
+
+   Push the changes to your account using:
+
+   ```bash
+   git push -u origin a-descriptive-name-for-my-changes
+   ```
+
+8. Once you are satisfied, go the webpage of your fork on GitHub. Click on "Pull request" to send your to the project maintainers for review.