Implement 'labels' option

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

docs/extends.md

@@ -321,8 +321,8 @@ expose:
   - "5000"
 ```
 
-In the case of `environment`, Compose "merges" entries together with
-locally-defined values taking precedence:
+In the case of `environment` and `labels`, Compose "merges" entries together
+with locally-defined values taking precedence:
 
 ```yaml
 # original service

docs/install.md

@@ -10,7 +10,7 @@ Compose with a `curl` command.
 
 ### Install Docker
 
-First, install Docker version 1.3 or greater:
+First, install Docker version 1.6 or greater:
 
 - [Instructions for Mac OS X](http://docs.docker.com/installation/mac/)
 - [Instructions for Ubuntu](http://docs.docker.com/installation/ubuntulinux/)

docs/yml.md

@@ -253,6 +253,24 @@ environment variables (DEBUG) with a new value, and the other one
 For more on `extends`, see the [tutorial](extends.md#example) and
 [reference](extends.md#reference).
 
+### labels
+
+Add metadata to containers using [Docker labels](http://docs.docker.com/userguide/labels-custom-metadata/). You can use either an array or a dictionary.
+
+It's recommended that you use reverse-DNS notation to prevent your labels from conflicting with those used by other software.
+
+```
+labels:
+  com.example.description: "Accounting webapp"
+  com.example.department: "Finance"
+  com.example.label-with-empty-value: ""
+
+labels:
+  - "com.example.description=Accounting webapp"
+  - "com.example.department=Finance"
+  - "com.example.label-with-empty-value"
+```
+
 ### net
 
 Networking mode. Use the same values as the docker client `--net` parameter.