update readme

Signed-off-by: William Casarin <jb55@jb55.com>

README.md

@@ -9,6 +9,18 @@ A cli util for creating nostr events
 
 `libsecp256k1` is the only dependency
 
+## Usage
+
+    usage: nostril [OPTIONS] <content>
+
+      OPTIONS
+
+          --dm <hex pubkey>               make an encrypted dm to said pubkey. sets kind and tags.
+          --envelope                      wrap in ["EVENT",...] for easy relaying
+          --kind <number>                 set kind
+          --created-at <unix timestamp>   set a specific created-at time
+          --sec <hex seckey>              set the secret key for signing, otherwise one will be randomly generated
+
 ## Examples
 
 Generate an event: