Move sample distribution to docs folder to avoid confusion.

docs/distro.md

@@ -1,5 +1,7 @@
 # Distribution Index
 
+You can use [Nebula](https://github.com/dscalzi/Nebula) to automate the generation of a distribution index.
+
 The distribution index is written in JSON. The general format of the index is as posted below.
 
 ```json