feat(README): add a modules section to the README

README.md

@@ -860,6 +860,71 @@ To do this just change the `-*-file` flags to `-peer-*-file`.
 
 If you are using SSL for server-to-server communication, you must use it on all instances of etcd.
 
+## Modules
+
+etcd has a number of modules that are built on top of the core etcd API.
+These modules provide things like dashboards, locks and leader election.
+
+### Dashboard
+
+An HTML dashboard can be found at `http://127.0.0.1:4001/mod/dashboard/```
+
+### Lock
+
+The Lock module implements a fair lock that can be used when lots of clients want access to a single resource.
+A lock can be associated with a name.
+The value is unique so if a lock tries to request a name that is already queued for a lock then it will find it and watch until that name obtains the lock.
+If you lock the same name on a key from two separate curl sessions they'll both return at the same time.
+
+Here's the API:
+
+**Retrieve the current value for the "customer1" lock.**
+
+```sh
+curl http://127.0.0.1:4001/mod/v2/lock/customer1
+```
+
+**Retrieve the current index for the "customer1" lock**
+
+```sh
+curl http://127.0.0.1:4001/mod/v2/lock/customer1?field=index
+```
+
+**Acquire a lock (with no value) for "customer1"**
+
+```sh
+curl -X POST http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60
+```
+
+**Acquire a lock for "customer1" that is associated with the value "bar"**
+
+```sh
+curl -X POST http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d value=bar
+```
+
+**Renew the TTL on the "customer1" lock for index 2**
+
+```sh
+curl -X PUT http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d index=2
+```
+
+**Renew the TTL on the "customer1" lock for value "customer1"**
+
+```sh
+curl -X PUT http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d value=bar
+```
+
+**Delete the "customer1" lock with the index 2**
+
+```sh
+curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?index=customer1
+```
+
+**Delete the "customer1" lock with the value "bar"**
+
+```sh
+curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?value=bar
+```
 
 ## Contributing