Add documentation for memprof.output_format ini setting (#103)

References #101

README.md

@@ -197,6 +197,14 @@ to PHP's own allocations.
 Note that when native tracking is enabled, the program will crash if a native
 library uses threads, because the underlying hooks are not thread safe.
 
+### Output format
+
+The output file format is defined with the `memprof.output_format` ini setting. The options are:
+- `callgrind` (default)
+- `pprof`
+
+**Note**: this may only be available when the extension is built from source (and not installed with pecl), as of https://github.com/arnaud-lb/php-memory-profiler/issues/101.
+
 ## Functions documentation
 
 ### memprof_enabled()