Add function documentation to README

README.md

@@ -1,3 +1,29 @@
 # libxconv
 Lightweight ISO-6937 to UTF-8 conversion library.
 
+## SYNOPSIS
+``
+int xconv(char *inbuf, char *outbuf, size_t outbuflen);
+``
+
+## DESCRIPTION
+
+The xconv() function converts a sequence of characters in ISO-6937
+encoding to UTF-8. 
+
+If a UTF-8 sequence is found within the input string, processing is aborted.
+
+## RETURN VALUE
+
+On successful conversion, the xconv() function returns the number of
+characters placed into _outbuf_.
+
+If no conversion is necessary (plain ASCII input string or a UTF-8 sequence
+was found) then the function returns 0. In this case, the state of the
+output buff is undefined and it should not be used.
+
+## NOTES
+
+This library was written to replace iconv() for a specific conversion task
+in a small embedded environment with little memory or CPU time available.
+