Fix604, Add reference to deployed cFE user's guide in README.md

Added a reference to the cFE user's guide (https://github.com/nasa/cFS/blob/gh-pages/cFE_Users_Guide.pdf) in the cFE README.md.

This reference was placed toward the top of the document, before the Version History section.

Resolves: nasa#604

README.md

@@ -6,7 +6,7 @@ This repository contains NASA's Core Flight Executive (cFE), which is a framewor
 
 This is a collection of services and associated framework to be located in the `cfe` subdirectory of a cFS Mission Tree. The Core Flight System is bundled at <https://github.com/nasa/cFS>, which includes build and execution instructions.
 
-The detailed cFE user's guide can be found at <https://github.com/nasa/cFS/blob/gh-pages/cFE_Users_Guide.pdf>.
+The detailed cFE user's guide can be viewed at <https://github.com/nasa/cFS/blob/gh-pages/cFE_Users_Guide.pdf>.
 
 ## Version History