Document OS compatibility restrictions for extensions

Added guidelines for 'os-families' and 'os-families-exclude' directives to restrict OS compatibility for extensions.

Author: macintoshplus

docs/extension-maintainers.md

@@ -241,6 +241,22 @@ change the behaviour of downloading the source package.
    * `{ExtensionName}-{Version}.tgz` (this is intended for backwards
      compatibility with PECL packages)
 
+##### `os-families` restrictions
+
+The `os-families` and `os-families-exclude` directive allow extention maintainers
+to restrict the Operating System compatibility.
+
+ * `os-families` An array of OS families to mark as compatible with the extension.
+   (e.g. `"os-families": ["windows"]` for an extension only available on Windows)
+ * `os-families-exclude` An array of OS families to mark as incompatible with the
+   extension. (e.g. `"os-families-exclude": ["windows"]` for an extension cannot
+   be installed available on Windows)
+
+The list of accepted OS families: "windows", "bsd", "darwin", "solaris", "linux",
+"unknown"
+
+> Note: only once of `os-families` and `os-families-exclude` can be defined.
+
 #### Extension dependencies
 
 Extension authors may define some dependencies in `require`, but practically,