Add FORMATTING.md

Author: winks

FORMATTING.md

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+# Formatting rules
+
+1. The format of this document is [Markdown](http://daringfireball.net/projects/markdown/)
+2. Everything in the actual specification document must be ASCII only.
+3. The markdown specification used seems to be *plain markdown* with tables and
+   code blocks (three backticks).
+4. Style
+  * Use spaces to indent, not tabs.
+  * Use of prefixed `##` is preferred for headers over underlined `===`
+  * Use of *inline* links `[example](http://example.org)` is preferred over
+    *reference* links `[example][xmpl]` unless used multiple times. This is to
+    allow for easier splitting and reorganization.
+  * Do not embed external images not hosted on php.net.
+  * Try to stick to 80 chars wide, if possible. This is not as strict as
+    in code, but still easier to read most of the times. This does only apply
+    to text, not to code examples or tables.