minor: note changed C driver requirements for the 3.1.x release series

Author: acmorrow

docs/content/mongocxx-v3/installation.md

@@ -38,7 +38,8 @@ have problems, please file a bug report via
 The mongocxx driver builds on top of the MongoDB C driver.
 
 * For mongocxx-3.2.x, libmongoc 1.9.2 or later is required.
-* For mongocxx-3.1.x, libmongoc 1.5.0 or later is required.
+* For mongocxx-3.1.[0-3], libmongoc 1.5.0 or later is required.
+* For mongocxx-3.1.4+, libmongoc 1.7.0 or later is required.
 * For mongocxx-3.0.x, we recommend the last 1.4.x version of libmongoc
 
 Unless you know that your package manager offers a high-enough version, you