added a readme based on the vimdoc introduction

README

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+arpeggio is a Vim plugin to define another type of key-mapping called
+arpeggio-key-mapping -- it consists of 2 or more keys ({lhs}) and it will be
+expanded to another key sequence ({rhs}) whenever all keys in {lhs} are
+simultaneously pressed.  For example:
+
+	Arpeggio inoremap jk  <Esc>
+	" OR
+	call arpeggio#map('i', '', 0, 'jk', '<Esc>')
+
+With the above definition, you can input <Esc> in Insert mode by pressing "j"
+and "k" simultaneously, while you can move the cursor by pressing "j" or "k"
+solely.
+
+The concept of this plugin is derived from Emacs' [key-chord.el][1], but there are the following differences:
+
+- Number of keys to be simultaneously pressed is unlimited.
+- Custom delay for each key is supported.
+  This is a generalization of [space-chord.el][2].
+
+[1] : http://www.emacswiki.org/emacs/key-chord.el
+[2] : http://www.emacswiki.org/emacs/space-chord.el