Put some warnings on the crypto stuff

README.md

@@ -60,7 +60,27 @@ actually serve up the app, which is entirely static content.
 Enabling encryption
 ===================
 
-To build a version of vector with the (experimental) support for end-to-end
+End-to-end encryption in Vector and Matrix is not yet considered ready for
-encryption, install the olm module with `npm i
+day-to-day use; it is experimental and should be considered only as a
-https://matrix.org/packages/npm/olm/olm-0.1.0.tgz` before running `npm
+proof-of-concept.
-start`. The olm library will be detected and used if available.
+
+To build a version of vector with support for end-to-end encryption, install
+the olm module with `npm i https://matrix.org/packages/npm/olm/olm-0.1.0.tgz`
+before running `npm start`. The olm library will be detected and used if
+available.
+
+To enable encryption for a room, type
+
+```
+/encrypt on
+```
+
+in the message bar in that room. Vector will then generate a set of keys, and
+encrypt all outgoing messages in that room. (Note that other people in that
+room will send messages in the clear unless they also `/encrypt on`.)
+
+Note that historical encrypted messages cannot currently be decoded - history
+is therefore lost when the page is reloaded.
+
+There is currently no visual indication of whether encryption is enabled for a
+room, or whether a particular message was encrypted.