Add library guide

docs/library-guide.md

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+# Guide to Writing Libraries in Ground
+
+Ground has a "use" keyword which allows you to import libraries written in Ground, executing the code, and importing functions for use. This makes building reproducable bits of code very easy.
+
+This is a guide of best practices which should be followed.
+
+## .grnd file extension
+
+The Ground interpreter will automatically append ".grnd" when you use a library. If you write `use "myLibrary"` Ground will look for "myLibrary.grnd". This is a must-do.
+
+## camelCase Function and File Names
+
+For consistency, please use camelCase (with a lower case first letter) when naming functions and file names.
+
+## Don't use spaces
+
+It is impossible to use spaces in Ground function names. Please do not use spaces in file names, even though it will work.
+
+## Use functions for most operations
+
+Where possible, create functions to do everything needed with your library. You can include some code for initialisation, but don't do the entirety of your operations outside of your functions.