CMake: Add missing docs for LV2URI argument

docs/CMake API.md

@@ -508,6 +508,17 @@ attributes directly to these creation functions, rather than adding them later.
 `AAX_IDENTIFIER`
 - The bundle ID for the AAX plugin target. Matches the `BUNDLE_ID` by default.
 
+`LV2URI`
+- This is a string that acts as a unique identifier for an LV2 plugin. If you make any incompatible 
+  changes to your plugin (remove parameters, reorder parameters, change preset format etc.) you MUST
+  change this value. LV2 hosts will assume that any plugins with the same URI are interchangeable.
+  By default, the value of this property will be generated based on the COMPANY_WEBSITE and
+  PLUGIN_NAME. However, in some circumstances, such as the following, you'll need to override the
+  default:
+  - The plugin name contains characters such as spaces that are invalid in a URI; or
+  - The COMPANY_WEBSITE omits the leading scheme identifier (http://); or
+  - There's no website associated with the plugin, so you want to use a 'urn:' identifier instead.
+
 `VST_NUM_MIDI_INS`
 - For VST2 and VST3 plugins that accept midi, this allows you to configure the number of inputs.
 

extras/Build/CMake/JUCEUtils.cmake

@@ -1180,9 +1180,9 @@ function(_juce_set_plugin_target_properties shared_code_target kind)
 
         get_target_property(JUCE_LV2URI "${shared_code_target}" JUCE_LV2URI)
 
-        if(NOT JUCE_LV2URI MATCHES "https?://.*")
+        if(NOT JUCE_LV2URI MATCHES "https?://.*|urn:.*")
             message(WARNING
-                "LV2URI should be well-formed with an 'http' prefix. "
+                "LV2URI should be well-formed with an 'http' or 'urn' prefix. "
                 "Check the LV2URI argument to juce_add_plugin.")
         endif()