Merge pull request public-apis#252 from davemachado/add-contributing-…

…guide

Add contributing guide

CONTRIBUTING.md

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+# Contributing to public-apis
+
+> While the masses of pull requests and community involvement is appreciated, some pull requests have been specifically 
+opened to market company APIs that offer paid solutions. This API list is not a marketing tool, but a tool to help the
+community build applications and use free, public APIs quickly and easily. Pull requests that are identified as marketing attempts will not be accepted.
+> 
+> Thanks for understanding! :)
+
+##Formatting
+
+Current API entry format:
+
+| API | Description | Auth | Link |
+| --- | --- | --- | --- |
+| API Title | Description of API | Does this API require authentication? * | Link to API webpage |
+
+Example entry:
+
+```
+| NASA | NASA data, including imagery | No | [Go!](https://api.nasa.gov) |
+```
+
+Currently, the only accepted inputs for this field are as follows:
+
+* `oAuth` - _the API supports oAuth_
+* `apiKey` - _the API uses a private key string/token for authentication - try and use the correct parameter_
+* `X-Mashape-Key` - _the name of the header which may need to be sent
+* No - _the API requires no authentication to run_
+
+Please continue to follow the alphabetical ordering that is in place.
+
+##Pull Request
+
+After you've created a branch on your fork with your changes, it's time to [make a pull request][pr-link]!
+
+Once you’ve submitted a pull request, the collaborators can review your proposed changes and decide whether or not to incorporate (pull in) your changes.
+
+###Pull Request Pro Tips
+
+* [Fork][fork-link] the repository and [clone][clone-link] it locally.
+Connect your local repository to the original `upstream` repository by adding it as a [remote][remote-link].
+Pull in changes from `upstream` often so that you stay up to date and so when you submit your pull request,
+merge conflicts will be less likely. See more detailed instructions [here][syncing-link].
+* Create a [branch][branch-link] for your edits.
+* Contribute in the style of the project as outlined above. This makes it easier for the collaborators to merge 
+and for others to understand and maintain in the future.
+
+###Open Pull Requests
+
+Once you’ve opened a pull request, a discussion will start around your proposed changes.
+
+Other contributors and users may chime in, but ultimately the decision is made by the collaborators.
+
+During the discussion, you may be asked to make some changes to your pull request.
+
+If so, add more commits to your branch and push them – they will automatically go into the existing pull request!
+
+[branch-link]: <http://guides.github.com/introduction/flow/>
+[clone-link]: <https://help.github.com/articles/cloning-a-repository/>
+[fork-link]: <http://guides.github.com/activities/forking/>
+[oauth-link]: <https://en.wikipedia.org/wiki/OAuth>
+[pr-link]: <https://help.github.com/articles/creating-a-pull-request/>
+[remote-link]: <https://help.github.com/articles/adding-a-remote/>
+[syncing-link]: <https://help.github.com/articles/syncing-a-fork>
+