Created Contributing (markdown)

Contributing.md

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+There are lots of opportunities to contribute to CyberChef. If you want ideas, take a look at any [Issues](https://github.com/gchq/CyberChef/issues) tagged with 'help wanted'.
+
+Before your contributions can be accepted, you must:
+
+ - Sign the [GCHQ Contributor Licence Agreement](https://github.com/gchq/Gaffer/wiki/GCHQ-OSS-Contributor-License-Agreement-V1.0)
+ - Push your changes to your fork.
+ - Submit a pull request.
+
+
+## Coding conventions
+
+* Indentation: Each block should consist of 4 spaces
+* Object/namespace identifiers: CamelCase
+* Function/variable names: underscore_lower_case
+* Constants: UNDERSCORE_UPPER_CASE
+* Source code encoding: UTF-8 (without BOM)
+* All source files must end with a newline
+* Line endings: UNIX style (\n)