Update 'Coding guidelines'

Coding-guidelines.md

@@ -1 +1,27 @@
-To be added
+# Coding Guidelines
+## Includes
+Header file includes shall be grouped by the categories below, with includes within each group being sorted alphabetically, and with empty lines seperating the groups:
+* The `gp_config` header, if required
+* Headers from the gplib project
+* C++ standard library headers
+* C standard library headers
+
+Headers from the gplib project shall be included using the quote form of the `include` directive.  
+Headers included conditionally shall be sorted as normal, and have no additional empty lines around them.
+### Example Code
+```cpp
+#pragma once
+
+#include "gp_config.hpp"
+
+#include "gp/algorithm/foreach.hpp"
+#ifndef NO_FP_MATH
+  #include "gp/math/fp_math.hpp"
+#endif
+#include "gp/math/integer_math.hpp"
+
+#include <atomic>
+#include <type_traits>
+
+#include <cstdint>
+```