Add commands description.

commands.md

+Setup
+=====
+
+    $ pod help setup
+      $ pod setup
+        This command will create a directory at `~/.cocoa-pods` which will hold your
+        spec-repos. In addition it will create a clone of the public ‘master’ spec-repo.
+
+
+Managing PodSpec files
+======================
+
+    $ pod help spec
+      pod spec create NAME
+        Creates a directory for your new pod, named `NAME', with a default
+        directory structure and accompanying `NAME.podspec'.
+
+      pod spec init NAME
+        Creates a PodSpec, in the current working dir, called `NAME.podspec'.
+        Use this for existing libraries.
+
+      pod spec edit NAME
+        Opens `NAME.podspec', from a local spec-repo, in the editor specified
+        in your shell environment by the `$EDITOR' variable. In case `NAME' is
+        omitted, it defaults to the PodSpec in the current working dir.
+
+      pod spec lint NAME
+        Validates `NAME.podspec' from a local spec-repo. In case `NAME' is
+        omitted, it defaults to the PodSpec in the current working dir.
+
+      pod spec push REMOTE
+        Validates `NAME.podspec' in the current working dir, copies it to the
+        local clone of the `REMOTE' spec-repo, and pushes it to the `REMOTE'
+        spec-repo. In case `REMOTE' is omitted, it defaults to `master'.
+
+
+Manage spec-repos
+=================
+
+    pod repo add NAME URL
+      Clones `URL' in the local spec-repos directory at ~/.cocoa-pods. The
+      remote can later be referred to by `NAME'.
+
+    pod repo update NAME
+      Updates the local clone of the spec-repo `NAME'.
+
+    pod repo change NAME URL
+      Changes the git remote of local spec-repo `NAME' to `URL'.
+
+    pod repo cd NAME
+      Changes the current working dir to the local spec-repo `NAME'.