docs(README): clarify planned features for v2.2.0 release

* Update the description of the upcoming `rpgp` backend
* Emphasize that `gnupg` will remain the default backend
* Note that `rpgp` may not cover all features initially but will improve over time

Author: saturneric

README.md

@@ -22,11 +22,12 @@ trustworthy to your privacy life.
 
 **Planned Next Release:** 
 
-- v2.2.0+: expected in the next few months, with a focus on introduce rust
-  support and add a new OpenPGP backend `rpgp` to have a better support for
-  OpenPGPv6 standard. The `gnupg` backend will be kept as the default one for a
-  long time, and the new `rpgp` backend will be optional for users who want to
-  try it out. 
+- v2.2.0+: expected in the next few months, with a focus on adding a new OpenPGP
+  backend `rpgp` to have a better support for OpenPGPv6 standard. The `gnupg`
+  backend will surely be kept as the default and the most important one, so the
+  new `rpgp` backend will be optional for users who want to try it out. It must
+  be noticed that the `rpgp` backend can not cover all the features of the
+  `gnupg` backend at the beginning, but it will be improved gradually. 
 
 ## Table of Contents