Add a Contributing guidelines and instructions

CONTRIBUTING.md

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+# Contributing to the Use Dai list
+
+Use Dai is a community driven effort! Contributions to the list are welcomed and encouraged from all.
+
+## Guidelines
+
+**To qualify for inclusion on the Use Dai list, a business or service should**:
+
+* Have a recently updated website explaining the nature of its business/service
+* Accept the Dai currency for payment in exchange for goods or services rendered, or otherwise allow Dai to be utilized as money, such as for lending or other financial products
+* Comply with all laws and statutes of the jurisdiction(s) in which it operates
+
+**Each contribution should**:
+
+* Conform to the following format: `Link - Short description of business or project`, for example:
+  * [Maker](https://cdp.makerdao.com/) - Provides loans in Dai against collateralized ETH`
+* Be **succinct** and **straightforward**. Avoid flowery language or marketing jargon.
+* Follow strict alphabetical ordering for sections, sub-sections, and entries
+* Include the project under the appropriate section & sub-section, adding them as needed
+* Update the table of contents if new a section or subsection was added
+
+## Instructions
+
+
+If you have something to the Use Dai list this is how you do it.
+
+You'll need a [GitHub account](https://github.com/join)!
+
+1. Access the list's GitHub page. For example: https://github.com/apbendi/use-dai
+2. Click on the `README` file: ![Step 2 Click on README.md](media/click-readme.png)
+3. Now click on the edit icon. ![Step 3 - Click on Edit](media/click-edit.png)
+4. You can start editing the text of the file in the in-browser editor. Make sure you follow guidelines above. You can use [GitHub Flavored Markdown](https://help.github.com/articles/github-flavored-markdown/). ![Step 4 - Edit the file](media/edit-readme.png)
+5. Say why you're proposing the changes, and then click on "Propose file change". ![Step 5 - Propose Changes](media/propose-changes.png)
+6. Submit the [pull request](https://help.github.com/articles/using-pull-requests/)!
+
+### Updating your Pull Request
+
+You may be asked to edit your Pull Request before it is included. This is normally due to spelling errors or because your PR didn't match the list guidelines.
+
+[Here](https://github.com/RichardLitt/knowledge/blob/master/github/amending-a-commit-guide.md) is a write up on how to change a Pull Request, and the different ways you can do that.

README.md

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 <p align="center">
 	<a href="https://makerdao.com/en/dai">What is Dai?</a>&nbsp;·&nbsp;
-	<a href="">Contribution Guide</a>&nbsp;·&nbsp;
+	<a href="CONTRIBUTING.md">Contribution Guide</a>&nbsp;·&nbsp;
 	<a href="https://twitter.com/intent/tweet?text=Checkout%20usedai.org%2C%20a%20community%20compiled%20list%20of%20everywhere%20you%20can%20use%20the%20decentralized%20%24DAI%20stablecoin">Tweet this</a>
 </p>