Upfront apologies

I apologize for not opening an issue first. Excuse:

- What I did

1. Fixed an issue where install.sh will not exit with error when supplied invalid arguments.
2. Added support to install.sh for --help argument.
3. Removed the dead link in the PR template Contributing guidance.
4. Added the PR template Contributing guidance to a Contributing section on the main README.

- How I did it

1. usage function called when invalid argument is provided to install.sh and script exits WITH error.
2. New (commented) usage function with new supported --help argument in install.sh which will exit WITHOUT error.
3. Deleted the sentence with the dead link in the PULL_REQUEST_TEMPLATE.md.
4. Copy/Paste/Edit the main contributing guidance from the PULL_REQUEST_TEMPLATE.md to the repo README.md.

- How to verify it

1. Exit install.sh with usage and error on invalid argument

sh install.sh --invalid

• BEFORE:

  

• AFTER:

  

2. Exit install.sh with usage and NO error on --help argument

sh install.sh --help

• BEFORE (same as above):

  

• AFTER:

  

Regression tests: install still works

• sh install.sh --dry-run

  

• sh install.sh

  

Linted and tested locally

• shellcheck:

  

• test:

  

README Contributing section

- Description for the changelog

(I know this says one line for the whole PR but traditionally changelogs have one line per THING and my PR has 2 distinct things)

• Fixes an issue where install.sh will not exit with error when supplied invalid arguments
• Enhances install.sh with --help argument to output usage statement

- A picture of a cute animal (not mandatory but encouraged)