added intro to signal to docs

docs/core-components.md

@@ -1,7 +1,7 @@
 # [Supe](../README.md) > Core Components
 
+## [Supervision](core-components/supervision.md)
+## [Signal](core-components/signal.md)
 ## [Noticeboard](core-components/noticeboard.md)
 ## [Mail](core-components/mail.md)
-## [Supervision](core-components/supervision.md)
-## Signal
-## Hook
+## Hook

docs/core-components/signal.md

@@ -0,0 +1,36 @@
+# [Supe](../../README.md) > [Core Components](../core-components.md) > Signal
+
+## Signal
+
+### About
+Signal is a simple lightweight way to send a message from one entity ( citizen, supervisor ) to another.
+
+Signal is Supe's lowest level inter-entity communication tool. While lightweight and fast, it lacks enough features to use directly for all inter-entity communication. No built-in mechanism for a signal's recipient to reply the sender.
+
+Despite these constraints, signal is a powerful solid building block for fancier messaging features and patterns. Supervision, mail and noticeboard all use signal to coordinate between two entities.
+
+If you're building custom or shareable Supe addons, Signal makes sense. Otherwise, you might get more value looking at communication tools that build on Signal and provide additional guarantees. 
+
+For example: 
+- Noticeboard is a signal that gets broadcasted to all entities, rather than a specific one.
+- Mail is signal that persists in the mailbox til the citizen processes and confirms it has been.
+
+`supervisor`
+```js
+var supe = require('supe')(),
+    signal_recipient = 'citizen-kane',
+    citizen = supe.get( signal_recipient );
+
+citizen.signal.send( 'HELLO-CITIZEN' )
+``` 
+
+`citizen`
+```js
+var citizen = require('supe');
+
+// to fellow citizen
+  citizen.signal.send({ to: 'citizen-kane' }, 'HELLO-FELLOW-CITIZEN' );
+
+// to supervisor
+  citizen.signal.send( 'HELLO-SUPE' );
+```