added introductory message and prerequisites to index/landing page

Author: Toby Hodges

index.md

@@ -2,9 +2,18 @@
 layout: lesson
 root: .
 ---
-FIXME: home page introduction
+Hello!
+
+This guide will introduce you to writing tool wrappers and workflows using the Common Workflow Language (CWL). This guide describes the current stable specification, version 1.0.
+
+Note: This document is a work in progress. Not all features are covered, yet.
 
 > ## Prerequisites
 >
-> FIXME
+> A text editor
+>
+> A CWL runner e.g. the [reference implementation](cwltool-repo) (see [the project homepage](cwl-home) for a full list)
 {: .prereq}
+
+[cwl-home]: http://www.commonwl.org
+[cwltool-repo]: https://github.com/common-workflow-language/cwltool