Add configure + make step to installation instructions

This compiles the `realpath` dynamic extension for bash which speeds up
symlink resolution. If the extension doesn't compile due to
cross-platform issues, rbenv will still work normally, although not as fast.

README.md

@@ -161,6 +161,13 @@ easy to fork and contribute any changes back upstream.
     $ git clone https://github.com/rbenv/rbenv.git ~/.rbenv
     ~~~
 
+    Optionally, try to compile dynamic bash extension to speed up rbenv. Don't
+    worry if it fails; rbenv will still work normally:
+
+    ~~~
+    $ cd ~/.rbenv && src/configure && make -C src
+    ~~~
+
 2. Add `~/.rbenv/bin` to your `$PATH` for access to the `rbenv`
    command-line utility.