Add jing for rng validation

Dockerfile

@@ -14,4 +14,5 @@ RUN chmod a+x /usr/local/bin/saxon \
         ant \
         ant-contrib \
         imagemagick \
+        jing \
     && npm install -g sass

README.md

@@ -7,4 +7,20 @@ Base docker image for building XML-based static sites in the DHIL
     * Apache ant and ant-contrib (for building)
     * saxon (for running XSLT)
     * sass (compiling SCSS -> CSS)
-    * imagemagick (routine image manipulation)
+    * imagemagick (routine image manipulation)
+    * jing (RelaxNG validation)
+* Working dir: `/var/www`
+
+After the image is pulled, it can be used like so:
+
+1. Running `ant` for a build in ${PROJECT_DIR}/build.xml:
+
+```bash
+docker run -v ${PROJECT_DIR}:/var/www dhilsfu/static-base:latest ant
+```
+
+2. Validating files in a directory using Jing:
+
+```bash
+ docker run -v $(pwd):/var/www dhilsfu/static-base:latest jing schema/schema.rng documents/*.xml
+```