skel/config.yml: Note that "engines:" must be merged

[cxw42]

Another thing that tripped me up, and that I would like to save others from. If config.yml has:

engines:
  template:
    ...
engines:
  session:
    ...

(i.e., two engines: keys), only the session-engine configuration will be loaded. The template-engine configuration will be silently discarded when the YAML file is loaded. This commit updates share/skel/config.yml to tell readers that you instead need to use:

engines:
  template:
    ...
  session:
    ...

This commit also adds a note about TT2 start_tag and end_tag being regexes, per #1526.

[cromedome]

Nice catch! I can see how that would be confusing. :+1:

[cromedome]

Merged, thanks!