Add a brief README for the mono-2.0 directory

Author: lambdageek

src/native/include/mono-2.0/README.md

@@ -0,0 +1,20 @@
+# The Mono Embedding API #
+
+The headers in this directory constitute the `mono-2.0` runtime embedding API.  They represent a
+contract between the runtime and clients.  Care must be taken to ensure that API and ABI
+compatibility is not broken.  For example: adding or removing function arguments, changing the
+return type, changing the layout of defined non-opaque structs, changing the calling convention, etc
+may potentially break existing clients and should not be undertaken lightly.
+
+Adding functions constitutes a contract between the runtime and potential future clients.  Adding a
+new public API function incurs a maintenance cost.  Care should be taken.
+
+## Unstable embedding API ##
+
+The headers `mono-private-unstable.h` in each subdirectory are an exception to the above guarantees.
+Functions added to these headers represent "work in progress" and may break API semantics or ABI
+compatability.  Clients using these functions are generally tightly coupled to the development of
+the runtime and take on responsibility for any breaking changes.
+
+
+