Change AOSP version requirement in README

and add note about older releases in branches.
diff --git a/README.md b/README.md
index bdb0b34..7b4b875 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,10 @@
 - Automatic daily backups that run in the background.
 
 ## Requirements
-- Android 10
+
+- Android 11
+
+For older versions of Android, check out [the branches](https://github.com/stevesoltys/seedvault/branches).
 
 ## Getting Started
 - Check out [the wiki](https://github.com/stevesoltys/seedvault/wiki) for information on building the application with