Document required library versions in README

Fixes #1517

Author: joto

README.md

@@ -56,6 +56,15 @@ Required libraries are
 * [Python](https://python.org/) (only for running tests)
 * [Psycopg](http://initd.org/psycopg/) (only for running tests)
 
+The following libraries are included in the `contrib` directory. You can build
+with other versions of those libraries (set the `EXTERNAL_*libname*` option to
+`ON`) but make sure you are using a compatible version:
+
+* [fmt](https://fmt.dev/) (>= 7.1.3)
+* [libosmium](https://osmcode.org/libosmium/) (>= 2.17.0)
+* [protozero](https://github.com/mapbox/protozero) (>= 1.6.3)
+* [rapidjson](https://rapidjson.org/) (>= 1.1.0)
+
 It also requires access to a database server running
 [PostgreSQL](https://www.postgresql.org/) 9.5+ and
 [PostGIS](http://www.postgis.net/) 2.2+.