added an opam .descr file

json_of_jsonm.descr

@@ -0,0 +1,11 @@
+json_of_jsonm_lib is a JSON encoder and decoder library that converts text to and from a
+`json` type. The library has the following features:
+
+* Uses jsonm to do the actual stream encoding and decoding
+* The `json` type is compatible with and a subset of yojson's `json` type
+* Provides both string and channel interfaces by default
+* The Json_string module provides a standard type `t` interface in addition to the
+  `json` type
+* Both `result` and exception functions are provided in most cases
+* The Json_encoder_decoder functor allows additional IO mechanisms, including Async,
+  to be defined easily.