Fix: How to contribute? (#214)

* Add README with project description + contribution guidelines.
* Updated webserver text in README
[skip ci]

README.md

@@ -0,0 +1,33 @@
+<p align="center">
+<a href="https://http://slackthemes.net" target="_blank"><img src="https://raw.githubusercontent.com/paracycle/slackthemes/master/source/favicon-32.png"></a>
+<br /><strong>Slack Themes</strong>
+</p>
+<p align="center">
+<a href="https://travis-ci.org/paracycle/slackthemes" target="_blank"><img src="https://travis-ci.org/paracycle/slackthemes.svg?branch=master"></a>
+</p>
+
+
+### About the project
+
+slackthemes.net is a platform for browsing and sharing themes for [Slack](https://slack.com/).
+It is not affiliated with [Slack](https://slack.com/) in any way or form; but it is a great tool, use it.
+
+
+### How to contribute (themes)
+
+1. Fork this project.
+2. Add your theme to the `data/themes.yml` file.
+3. Create a pull request.
+
+### How to contribute (core)
+
+1. Fork this project.
+2. Install dependencies using `bundle install`
+3. Make changes.
+4. Build the project using `bundle exec middleman build`
+5. You can [run a local webserver](https://developer.mozilla.org/en-US/docs/Learn/Common_questions/set_up_a_local_testing_server) to test the project. (make sure to use the `./build` folder as your DocumentRoot).
+6. Create a pull request.
+
+### Contributors
+
+* [Contributors](https://github.com/paracycle/slackthemes/graphs/contributors)