Add README info about iText dependencies for the SNAPSHOT version.

See http://stackoverflow.com/questions/41531997/error-building-itext-examples-sandbox-5-5-11-snapshot

README.md

@@ -1,2 +1,22 @@
 iText 5 Sandbox
----
+---------------
+
+To build these examples:
+
+* Either checkout the latest release tag. The jars you need are from the latest official iText release and are published on the [Maven Central][1] repository.
+* Or checkout the `develop` branch. This depends on the current `HEAD` of iText. You have two options:
+  * Use the [iText Artifactory server][2] for `SNAPSHOT` builds. Add this to your `pom.xml`:
+  ```
+  <repositories>
+    <repository>
+      <id>itext</id>
+      <name>iText Repository - snapshots</name>
+      <url>https://repo.itextsupport.com/snapshot/</url>
+    </repository>
+  </repositories>
+  ```
+  * [Clone the iText repository][3] and build _everything_ yourself.
+
+[1]: https://search.maven.org/
+[2]: https://repo.itextsupport.com/snapshot/
+[3]: https://github.com/itext/itextpdf

pom.xml

@@ -129,6 +129,14 @@
         </plugins>
     </build>
 
+    <repositories>
+      <repository>
+        <id>itext</id>
+        <name>iText Repository - snapshots</name>
+        <url>https://repo.itextsupport.com/snapshot/</url>
+      </repository>
+    </repositories>
+
     <profiles>
         <profile>
             <id>java8-disable-strict-javadoc</id>