Add more explicit notice regarding Snappy compression

Fixes dain#119

Signed-off-by: Stavros Ntentos <[email protected]>

README.md

@@ -172,3 +172,11 @@ factory.destroy(new File("example"), options);
 
 * [ActiveMQ Apollo](http://activemq.apache.org/apollo/): Defaults to using leveldbjni, but falls 
   back to this port if the jni port is not available on your platform.
+
+## Notes
+### Snappy compression
+
+In order to use this library with leveldb databases using Snappy compression,
+you need to also include one of the following libraries:
+* `org.xerial.snappy:snappy-java:1.1.8.4`
+* `org.iq80.snappy:snappy:0.4`

leveldb/src/main/java/org/iq80/leveldb/util/Snappy.java

@@ -231,6 +231,10 @@ public static boolean available()
     public static void uncompress(ByteBuffer compressed, ByteBuffer uncompressed)
             throws IOException
     {
+        if (SNAPPY == null) {
+            throw new RuntimeException("This is a compressed database, and snappy library is not found in your classpath!" +
+                                               "See more: https://github.com/dain/leveldb/#snappy-compression");
+        }
         SNAPPY.uncompress(compressed, uncompressed);
     }