Add syntax documentation

syntax.md

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+# Slate Syntax
+
+## Headers
+
+For headers:
+
+    # Level 1 Header
+    ## Level 2 Header
+    ### Level 3 Header
+
+Note that only level 1 and 2 headers will appear in the table of contents.
+
+## Paragraph Text
+
+For normal text, just type your paragraph on a single line.
+
+  This is some paragraph text. Exciting, no?
+
+Make sure the lines above below your paragraph are empty.
+
+## Code Samples
+
+For code samples:
+
+    ```ruby
+    # This is some Ruby code!
+    ```
+
+    ```python
+    // This is some Python code!
+    ```
+
+Code samples will appear in the dark area to the left of the main text. We recommend positioning code samples right under headers in your markdown file.
+
+For the full list of supported languages, see [rouge](http://rouge.jayferd.us/demo).
+
+## Code Annotations
+
+For code annotations:
+
+    ```ruby
+    # This is some Ruby code!
+    ```
+
+    > This is a code annotation. It will appear in the area to the right, next to the code samples.
+
+    ```ruby
+    # This is some more Ruby code!
+    ```
+
+Code annotations are essentially the same thing as paragraphs, but they'll appear in the area to the right along with your code samples.
+
+## Tables
+
+Slate uses PHP Markdown Extra style tables:
+
+    Table Header 1 | Table Header 2 | Table Header 3
+    -------------- | -------------- | --------------
+    Row 1 col 1 | Row 1 col 2 | Row 1 col 3
+    Row 2 col 1 | Row 2 col 2 | Row 2 col 3
+
+Note that the pipes do not need to line up with each other on each line.
+
+If you don't like that syntax, feel free to use normal html `<table>`s directly in your markdown.
+
+## Formatting Text:
+
+    This text is ***bold***, this is **italic**, this is an `inline code block`.
+
+You can use those formatting rules in code annotations, tables, paragraphs, lists, wherever.
+
+## Lists
+
+    1. This
+    2. Is
+    3. An
+    4. Ordered
+    5. List
+
+    * This
+    * Is
+    * A
+    * Bullet
+    * List
+
+## Links
+
+    This is an [internal link](#error-code-definitions), this is an [external link](http://google.com).