Update template modification docs

[jimschubert]

Updates some confusing wording, specifies naming conventions, and links
to external new.sh for contributed templates (rather than modified
templates).

This makes some additional adjustments suggested in #283.

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

[jimschubert]

@wing328 I'm going to merge this PR because it's docs and I think it's necessary to clarify a few small points in the document. I'm merging because I opened the PR 3 hours ago and Shippable is still running, which I think will cancel once merged.

[jmini]

For the moment we do not do anything with the docs files ( *.md files) in our repo (checks, publish doc, ...).

So not waiting for the CI jobs is OK.

Thank you a lot for improving the docs!