Document msg module customization method

[skliper]

Is your feature request related to a problem? Please describe.
No documentation on how to customize message module

Describe the solution you'd like
Add documentation - overview, suggestions, etc

Include cmake steps to replace:

# The "MISSION_CORE_MODULES" will be built and statically linked as part
# of the CFE core executable on every target.  These can be used to amend
# or override parts of the CFE core on a mission-specific basis.
list(APPEND MISSION_MODULE_SEARCH_PATH "prj_cfe")
list(REMOVE_ITEM MISSION_CORE_MODULES "msg")
list(APPEND MISSION_CORE_MODULES "custom_msg")

Note this is a "power user" option and not generally recommended unless absolutely required by the mission.

Describe alternatives you've considered
None.

Additional context
None.

Requester Info
Jacob Hageman - NASA/GSFC