Add basic usage example in usage.rst

Author: gmelikov

doc/source/usage.rst

@@ -2,6 +2,39 @@
 Usage
 ========
 
-To use etcd3-gateway in a project::
+You can find examples in ``etcd3gw/examples`` and look at ``etcd3gw/client.py``.
 
-    import etcd3gw
+Basic usage example::
+
+    from etcd3gw.client import Etcd3Client
+ 
+    client = Etcd3Client(host='localhost', port=2379)
+
+    # Put key
+    client.put(key='foo', value='bar')
+
+    # Get key
+    client.get(key='foo')
+
+    # Get all keys
+    client.get_all()
+
+
+    # Create lease and use it
+    lease = client.lease(ttl=100)
+
+    client.put(key='foo', value='bar', lease=lease)
+
+    # Get lease keys
+    lease.keys()
+
+    # Refresh lease
+    lease.refresh()
+
+
+    # Use watch
+    watcher, watch_cancel = client.watch(key='KEY')
+
+    for event in watcher: # blocks until event comes, cancel via watch_cancel()
+        print(event)
+        # modify event: {u'kv': {u'mod_revision': u'8', u'version': u'3', u'value': 'NEW_VAL', u'create_revision': u'2', u'key': 'KEY', u'lease': u'7587847878767953426'}}