Document pre-requisites for `setup_boostbook.sh`. Fixes #3292

Merged revisions 55224 via svnmerge from 
https://svn.boost.org/svn/boost/trunk

[SVN r55277]

tools/boostbook

@@ -1 +1 @@
-Subproject commit 1001c156ef7e5df27d892f5ae180485394e9e2ec
+Subproject commit 286d485b53c8cca7152193bec89a63f3d70b8d8e