Rewrite installation instructions

Closes vim-ruby#40

INSTALL.markdown

@@ -0,0 +1,36 @@
+Installation
+============
+
+In general, your favorite method work.  Here are some options.
+
+With pathogen.vim
+-----------------
+
+Install [pathogen.vim](https://github.com/tpope/vim-pathogen),
+then copy and paste:
+
+    git clone git://github.com/vim-ruby/vim-ruby.git ~/.vim/bundle/vim-ruby
+
+With Vundle
+-----------
+
+Install [Vundle](https://github.com/gmarik/vundle), then add the
+following to your vimrc:
+
+    Bundle 'vim-ruby/vim-ruby'
+
+With patience
+-------------
+
+Wait for an upgrade to Vim and install it.  Vim ships with the latest
+version of vim-ruby at the time of its release.  (Remember this when
+choosing another installation method.  The version you download will
+supersede the version that ships with Vim, so you will now be
+responsible for keeping it up-to-date.)
+
+Manually
+--------
+
+[Download](https://github.com/vim-ruby/vim-ruby/archives/master) and
+extract an archive, and run `bin/vim-ruby-install.rb` to copy the
+relevant files to `~/.vim`.