Add documentation on how Android encryption works.

Change-Id: I20f55eca5e6043e0d8dd3eef4d3f17f5587c56af
diff --git a/src/tech/encryption/index.md b/src/tech/encryption/index.md
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+<!--
+   Copyright 2010 The Android Open Source Project 
+
+   Licensed under the Apache License, Version 2.0 (the "License"); 
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+-->
+
+# Encryption Technical Information #
+
+Encryption on Android uses the dm-crypt layer in the Linux kernel.  Read the
+detailed description of how it is tied into the Android system and what must
+be done on a new device to get this feature working.
+