add docker link

Author: idealeer

README.md

@@ -17,7 +17,7 @@ The latest stable release of XMap is version 1.1.3 and supports Linux, macOS, an
 
 **Instructions on building XMap from source** can be found in [INSTALL](https://github.com/idealeer/xmap/blob/master/INSTALL.md).
 
-**Installing from docker**: `docker pull liii/xmap:latest`
+**Installing from [docker](https://hub.docker.com/r/liii/xmap)**: `docker pull liii/xmap:latest`
 
 Usage
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