Add mention of RUBY_MAKE_INSTALL_OPTS in README

README.md

@@ -119,9 +119,9 @@ You can set certain environment variables to control the build process.
 * `MAKE` lets you override the command to use for `make`. Useful for specifying
   GNU make (`gmake`) on some systems.
 * `MAKE_OPTS` (or `MAKEOPTS`) lets you pass additional options to `make`.
-* `RUBY_CONFIGURE_OPTS` and `RUBY_MAKE_OPTS` allow you to specify configure and
-  make options for buildling MRI. These variables will be passed to Ruby only,
-  not any dependent packages (e.g. libyaml).
+* `RUBY_CONFIGURE_OPTS`, `RUBY_MAKE_OPTS` and `RUBY_MAKE_INSTALL_OPTS` allow
+  you to specify configure and make options for buildling MRI. These variables
+  will be passed to Ruby only, not any dependent packages (e.g. libyaml).
 
 ### Applying patches to Ruby before compiling