Add README detail around file format compatibility

Author: TJC

README.md

@@ -83,9 +83,20 @@ Stream-based compressor/decompressor `SnappyOutputStream`/`SnappyInputStream` ar
  * See also [Javadoc API](https://oss.sonatype.org/service/local/repositories/releases/archive/org/xerial/snappy/snappy-java/1.1.3-M1/snappy-java-1.1.3-M1-javadoc.jar/!/index.html)
 
 #### Compatibility Notes
- * `SnappyOutputStream` and `SnappyInputStream` use `[magic header:16 bytes]([block size:int32][compressed data:byte array])*` format. You can read the result of `Snappy.compress` with `SnappyInputStream`, but you cannot read the compressed data generated by `SnappyOutputStream` with `Snappy.uncompress`. Here is the data format compatibility matrix:
+
+The original Snappy format definition did not define a file format. It later added
+a "framing" format to define a file format, but by this point major software was
+already using an industry standard instead -- represented in this library by the
+`SnappyOutputStream` and `SnappyInputStream` methods.
+
+For interoperability with other libraries, check that compatible formats are used.
+Note that not all libraries support all variants.
+
+ * `SnappyOutputStream` and `SnappyInputStream` use `[magic header:16 bytes]([block size:int32][compressed data:byte array])*` format. You can read the result of `Snappy.compress` with `SnappyInputStream`, but you cannot read the compressed data generated by `SnappyOutputStream` with `Snappy.uncompress`.
  * `SnappyHadoopCompatibleOutputStream` does not emit a file header but write out the current block size as a  preemble to each block
 
+#### Data format compatibility matrix:
+
 | Write\Read      | `Snappy.uncompress`   | `SnappyInputStream`  | `SnappyFramedInputStream` | `org.apache.hadoop.io.compress.SnappyCodec` |
 | --------------- |:-------------------:|:------------------:|:-----------------------:|:-------------------------------------------:|
 | `Snappy.compress` | ok | ok | x | x |