Add note on Function naming (#321)

* Add note on Function naming

* Review feedback - function and value names

* Fix Apostrophie typo

Author: milesfrain

language/Syntax.md

@@ -97,6 +97,15 @@ multiply x y = x * y
 
 Type signatures are not required for top-level declarations in general, but is good practice to do so. See the section on types for more details.
 
+## Function and Value names
+
+Function and value names must start with a lowercase letter ([unicode category](https://en.wikipedia.org/wiki/Unicode_character_property#General_Category) `Ll`) or underscore `_`. The following characters may be any number of:
+* Letter: Unicode category `L`
+* Mark: Unicode category `M`
+* Number: Unicode category `N`
+* Underscore: `_`
+* Apostrophe: `'`
+
 ## Function application
 
 Function application is indicated by just the juxtaposition of a function with its arguments: