Add documentation for 'constant numerical expressions' parameters.

doc/docs/contributing.md

@@ -37,6 +37,7 @@ If you wish to add a function to any of these libraries or if you plan to add a
 * Functions must be organized by sections.
 * Any new library should at least `declare` a `name` and a `version`.
 * Any new library has to use a prefix declared in the header section with the following kind of syntax: `Its official prefix is 'qu'` (look at an existing library to follow the exact syntax).
+* Some functions use parameters that are [constant numerical expressions](https://faustdoc.grame.fr/manual/syntax/#constant-numerical-expressions). The convention is to label them in *capital letters* and document them preferably to be *constant numerical expressions* (or *known at compile time* in existing libraries).
 * The comment based markdown documentation of each library must respect the following format (open the source code of any of the libraries for an example):
 
 ```