Modify docs/index to point to useful materials

Signed-off-by: David A. Wheeler <[email protected]>

Author: david-a-wheeler

docs/index.md

@@ -1,3 +1,12 @@
 # lfd121.openssf.org
 
-This is lfd121.openssf.org.
+If you want to *learn* using the online tutorials, you're in the *wrong place*.
+Instead, *please* go to the
+**[Developing Secure Software (LFD121) entry page](https://training.linuxfoundation.org/training/developing-secure-software-lfd121/)**
+so you can learn how to develop secure software.
+It's free, and its certificate of completion is free.
+
+The [OpenSSF page about this course](https://openssf.org/training/courses/) provides some broader context about the course.
+The learning material also available via [edX](http://edx.org/professional-certificate/linuxfoundationx-secure-software-development-fundamentals?utm_medium=partner-marketing&utm_source=affiliate&utm_campaign=openssf&utm_content=openssforg-securedevelopmentpc). but the edX certificate of completion is not free.
+
+However, if you want to view the course material as a book, you're in the right place! Please view the [*Developing Secure Software (LFD121) course material as a book*](./lfd121).