pkgconfig: Document url keyword

Closes #2272

docs/markdown/Pkgconfig-module.md

@@ -34,6 +34,7 @@ keyword arguments.
 - `requires` list of strings to put in the `Requires` field
 - `requires_private` list of strings to put in the `Requires.private`
   field
+- `url` a string with a url for the library
 - `variables` a list of strings with custom variables to add to the
   generated file. The strings must be in the form `name=value` and may
   reference other pkgconfig variables,

mesonbuild/modules/pkgconfig.py

@@ -121,7 +121,7 @@ class PkgConfigModule(ExtensionModule):
 
     @permittedKwargs({'libraries', 'version', 'name', 'description', 'filebase',
                       'subdirs', 'requires', 'requires_private', 'libraries_private',
-                      'install_dir', 'extra_cflags', 'variables'})
+                      'install_dir', 'extra_cflags', 'variables', 'url'})
     def generate(self, state, args, kwargs):
         if len(args) > 0:
             raise mesonlib.MesonException('Pkgconfig_gen takes no positional arguments.')