README: adding some usage instructions

Author: László Vaskó

README.md

@@ -1,2 +1,31 @@
 # openconnect-sso
 Wrapper script for OpenConnect supporting Azure AD (SAMLv2) authentication
+
+## TL; DR
+```bash
+$ pip install openconnect-sso-$VERSION.whl
+$ openconnect-sso --server vpn.server.com/group
+```
+
+## Configuration
+If you want to save credentials and get them automatically
+injected in the web browser:
+```bash
+$ openconnect-sso --server vpn.server.com/group --user [email protected]
+Password ([email protected]): 
+[info     ] Authenticating to VPN endpoint ...
+```
+
+User credentials are automatically saved to the users login keyring (if available).
+
+If you already have Cisco Anyconnect set-up, then `--server` argument is optional.
+Also, the last used `--server` address is saved between sessions so there is no need
+to always type in the same arguments:
+
+```bash
+$ openconnect-sso
+[info     ] Authenticating to VPN endpoint ...
+```
+
+Configuration is saved in `$XDG_CONFIG_HOME/openconnect-sso/config.toml`. On typical
+Linux installations it is located under `$HOME/.configopenconnect-sso/config.toml`