Add CONTRIBUTING

CONTRIBUTING.md

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+# Contributing
+The goal is to be a repository for the original Apollo 11 source
+code. 
+
+As such, [Pull Requests][0] are welcome for any issues identified 
+between the transcriptions in this repository and the GAP scans for
+[Luminary 099][1] and [Comanche 055][2], as well as any missing 
+files.
+
+## How to Contribute
+
+## Guidelines
+### Comments
+- Comments in the transcribed code should match 1:1 with the 
+respective image. 
+  - This could involve creating a deliberate typo or removing/adding
+  an entire comment.
+
+### Line breaks
+- Line breaks **with** `R0000` in column 1 should match 1:1.
+- Line breaks **without** `R0000` in column 1 should only contain 1
+to 2 blank lines in a row. Lines with `R0000` do not count towards
+this.
+  - If there are more than 2 blank lines breaks, strip the extra 
+  line breaks.
+  - Lines with `R0000` in column 1 do not count towards this number.
+  - In the source images, these were created by an unprinted digit
+  in column 8. A 2 there forced a double space (single blank line)
+  and a 3 forced a triple space (double blank line). Values 4-8 were
+  defined but never used. Read more about it in #159
+
+## Formatting
+
+[AGC Assembly][4] for Sublime Text 3 and [Language AGC][5] for Atom
+ will ensure you're using the correct formatting automatically.
+
+[AGC Assembly][3] for Visual Studio Code provides language-specific
+settings overrides to add to your user settings [here][6].
+
+- Use tab indentation
+- Use tab width of 8
+- Trim trailing whitespace
+
+## Useful Extensions
+
+GitHub has syntax support for the AGC assembly language built-in.
+Unfortunately your code editor will not. Luckily there is a language
+extension that provides syntax highlighting for the following 
+editors:
+- [Visual Studio Code][3]
+- [Sublime Text 3][4]
+- [Atom][5]
+
+[0]:https://github.com/chrislgarry/Apollo-11/pull/new/master
+[1]:http://www.ibiblio.org/apollo/ScansForConversion/Luminary099/
+[2]:http://www.ibiblio.org/apollo/ScansForConversion/Comanche055/
+[3]:https://github.com/wopian/agc-assembly
+[4]:https://github.com/jimlawton/AGC-Assembly
+[5]:https://github.com/Alhadis/language-agc
+[6]:https://github.com/wopian/agc-assembly#user-settings
+[7]:https://github.com/chrislgarry/Apollo-11/issues/159