docs/introduction.rst: add feature Encryption, replace LZ4 with Zstandard

docs/introduction.rst

@@ -46,13 +46,18 @@ Main features
 :Performance: The whole software stack is written in :term:`Rust`,
    which provides high speed and memory efficiency.
 
-:Compression: Ultra fast `LZ4`_ compression is able to compress
+:Compression: Ultra fast Zstandard_ compression is able to compress
    several gigabytes of data per second.
 
+:Encryption: Backups can be encrypted at client side using AES-256 in
+   GCM_ mode. This authenticated encryption mode (AE_) provides very
+   high performance on modern hardware.
+
 :Open Source: No secrets. You have access to the whole source tree.
 
 :Support: Commercial support options available from `Proxmox`_.
 
+
 Why Backup?
 -----------
 
@@ -78,6 +83,13 @@ required by law.
 - restore tests. to make sure backup/restore works
 
 
+Software Stack
+--------------
+
+
+License
+-------
+
 
 
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