Add documentation on how Android encryption works.

Change-Id: I20f55eca5e6043e0d8dd3eef4d3f17f5587c56af
diff --git a/src/tech/sidebar.md b/src/tech/sidebar.md
index 6330fad..11c01b3 100644
--- a/src/tech/sidebar.md
+++ b/src/tech/sidebar.md
@@ -2,3 +2,4 @@
 - [Dalvik](/tech/dalvik/index.html)
 - [Bluetooth](/tech/bluetooth/index.html)
 - [NFC](/tech/nfc/index.html)
+- [Encryption](/tech/encryption/index.html)