Update docs

Close #5 #12

README.md

@@ -1,5 +1,34 @@
-The "which" util from npm's guts.
+# which
+
+Like the unix `which` utility.
 
 Finds the first instance of a specified executable in the PATH
 environment variable.  Does not cache the results, so `hash -r` is not
 needed when the PATH changes.
+
+## USAGE
+
+```javascript
+var which = require('which')
+
+// async usage
+which('node', function (er, resolvedPath) {
+  // er is returned if no "node" is found on the PATH
+  // if it is found, then the absolute path to the exec is returned
+})
+
+// sync usage
+// throws if not found
+var resolved = which.sync('node')
+
+// Pass options to override the PATH and PATHEXT environment vars.
+which('node', { path: someOtherPath }, function (er, resolved) {
+  if (er)
+    throw er
+  console.log('found at %j', resolved)
+})
+```
+
+## OPTIONS
+
+If you pass in options, then `path` and `pathExt` are relevant.