Add installation instructions for Arch Linux

Closes https://github.com/sstephenson/ruby-build/issues/526.

README.md

@@ -55,6 +55,20 @@ Or, if you would like to install the latest development release:
 
     brew install --HEAD ruby-build
 
+### Installing Ruby 2.x on Arch Linux
+
+If you are trying to install Ruby 2.x on Arch Linux (or similar), you may need
+to manually specify the `readline` location manually.
+
+    RUBY_CONFIGURE_OPTS=--with-readline-dir="/usr/lib/libreadline.so" rbenv install 2.1.0
+
+This should no longer be necessary once this [issue][] is resolved in Ruby.
+Many thanks to [**@ahacking**][ahacking] for [investigating this issue and
+contributing this workaround][workaround].
+
+[issue]: https://bugs.ruby-lang.org/issues/9630
+[ahacking]: https://github.com/ahacking
+[workaround]: https://github.com/sstephenson/ruby-build/issues/526
 
 ## Usage