build message if kernel source isn't installed

Previously no check was made to see if the kernel
source was installed before trying to build the modules.
And if it wasn't, a bunch of unhelpful messages would
be displayed.  Now it will display a single helpful
message indicating that the source isn't installed.
cleaned up hello documentation, added README

The wording in the main document for the hello module
has been cleaned up.  To help new users a README was
added which describes how to use the module and also
points to the main documentation.
more cleanup of hello documentation
general documentation review/cleanup

The main documentation has been reviewed and
all the examples checked.  Many minor wording
changes and typos were fixed.
Merge branch 'master' into doc-build
new doc build