Document how to find the jar library versions for carrier/geocoding/prefixmapper (#1669)

FAQ.md

@@ -255,6 +255,26 @@ The metadata binary files can be generated using the ant build rules
 file](https://github.com/googlei18n/libphonenumber/blob/master/java/build.xml)
 itself.
 
+### Which versions of the Maven jars should I use?
+
+When possible, use the [latest
+version](https://github.com/googlei18n/libphonenumber/releases) of
+libphonenumber.
+
+For the other Maven artifacts, to find the version corresponding to a given
+version of libphonenumber, follow these steps:
+
+*   Go to the versioned GitHub tag, e.g.
+    https://github.com/googlei18n/libphonenumber/find/v8.3.3
+*   Type `pom.xml`. This will surface all the `pom.xml` files as they were
+    released at the chosen tag.
+*   Find the version you care about in the corresponding `pom.xml` file. Look
+    for `<version>` in the top-level `project` element. For example, to find the
+    version of the carrier jar corresponding to libphonenumber 8.3.3, open
+    `java/carrier/pom.xml` at v8.3.3's search results. This is `1.56`.
+*   If you depend on the carrier or geocoder jar, you also need to depend on
+        the prefixmapper jar.
+
 ### How do I load libphonenumber resources in my Android app?
 
 #### System considerations