Update standard with robert's now proposal

doc/coding_standard.txt

@@ -13,6 +13,8 @@ must do's:
 		const wherever possible (const-correctness)
 		pass by pointer or by reference
 		prefer reference over pointer
+		keep implementation in source files with template and constexpr as exceptions
+		keep template/constexpr implementations in .tpp files alongside the .hpp files
 	functions:
 		50 max length
 		comments max 2 lines
@@ -76,3 +78,4 @@ no-no's:
 	`class` in template parameter list
 	struct keyword before an object declaration
 		struct game_state* gl; //bad boi
+	excess include directives