Create the prerequisites documentation

The prerequisites.md file documentation is based on the online version:
https://docs.cachethq.io/docs/prerequisites

docs/setup/prerequisites.md

@@ -0,0 +1,40 @@
+# Prerequisites
+
+To start using Cachet, you'll need some prerequisites.
+
+## Application Prerequisites
+
+You'll need at least the following installed on your server:
+
+- PHP 5.5.9, you'll also need `ext-gd`, `ext-simplexml`, `mcrypt`,
+  `ext-xml`, `ext-mbstring` and `ext-tokenizer` installed.
+- [Composer][1]
+- APCu or Redis for caching.
+- A database driver for your DB, such as MySQL, PostgreSQL or SQLite.
+- Git
+
+> **SQLite**
+> Whilst we support SQLite, we advise not using it for status pages
+> with a high amount of traffic.
+
+> **MySQL Timezone Info**
+> Ensure your MySQL database has been updated with the correct timezone
+> information. This will ensure that metrics are shown
+> correctly: [https://dev.mysql.com/doc/refman/5.7/en/time-zone-support.html][2]
+
+## Developer prerequisites
+
+If you are looking to contribute to Cachet, thank you! Your help is really
+appreciated! You may need some extra dependencies; depending on what you're looking for.
+
+Our CSS is compiled from SCSS, so to compile this you will need the following:
+
+- Node.js
+- NPM
+- Bower
+- Gulp
+
+
+
+[1]: https://getcomposer.org/
+[2]: https://dev.mysql.com/doc/refman/5.7/en/time-zone-support.html