feat(Documentation/api): add some intro material

add some intro material and normalize the headers to the rest of the
coreos/docs.

Documentation/api.md

@@ -1,6 +1,6 @@
-## etcd API
+# etcd API
 
-### Running a Single Machine Cluster
+## Running a Single Machine Cluster
 
 These examples will use a single machine cluster to show you the basics of the etcd REST API.
 Let's start etcd:
@@ -13,6 +13,10 @@ This will bring up etcd listening on port 4001 for client communication and on p
 The `-data-dir machine0` argument tells etcd to write machine configuration, logs and snapshots to the `./machine0/` directory.
 The `-name machine` tells the rest of the cluster that this machine is named machine0.
 
+## Key Space Operations
+
+The primary API of etcd is hierarchical key space.
+There are directories and keys which are generically referred to as "nodes".
 
 ### Setting the value to a key