DOCSP-49294 Add MDB Extension section to compat page (#3357)

Author: stephmarie17

Diff for: docs/compatibility.txt

@@ -28,3 +28,15 @@ the {+odm-short+} that you can use together.
 To find compatibility information for unmaintained versions of the {+odm-short+},
 see `Laravel Version Compatibility <{+mongodb-laravel-gh+}/blob/3.9/README.md#installation>`__
 on GitHub.
+
+PHP Driver Compatibility
+------------------------
+
+To use {+odm-long+} v4.9 or later, you must install v1.21 of the
+{+php-library+} and {+php-extension+}.
+
+.. important:: {+php-extension+} v2.0 Compatibility
+
+   If you upgrade the {+php-extension+} to v2.0, you must also upgrade
+   {+odm-long+} to v4.9.0. {+odm-long+} v4.9.0 still supports v1.x
+   versions of the extension.