Add UPGRADING note

Signed-off-by: Thomas Farr <[email protected]>

UPGRADING.md

@@ -1,6 +1,9 @@
 # UPGRADING
 
-## [UPGRADING 2.x to 3.0]
+## Upgrading 2.x to 3.0
+### URL Path Encoding
+- The default URL path encoding has been changed to be more conservative. Previously the `!`, `$`, `&`, `'`, `(`, `)`, `*`, `+`, `,`, `;`, `=`, `@` and `:` characters were left un-encoded, they will now be percent-encoded. If you require the previous behavior you can specify the `org.opensearch.path.encoding=HTTP_CLIENT_V4_EQUIV` system property.
+
 ### SearchAfter of SearchRequest type
 - Changed SearchAfter of SearchRequest type to FieldValue instead of String ([#769](https://github.com/opensearch-project/opensearch-java/pull/769))
 - Consider using `FieldValue.of` to make string type values compatible.