Clarify how to write links in Markdown

[eatyourgreens]

Talk discussion about writing links in Markdown implies the instructions are confusing

Describe your changes.
Removes the 'URL shortening' wording and tries to clarify internal vs. external links

Review Checklist

• Does it work in all major browsers: Firefox, Chrome, Edge, Safari?
• Does it work on mobile?
• Can you rm -rf node_modules/ && npm install and app works as expected?
• Did you deploy a staging branch? https://markdown-help-changes.pfe-preview.zooniverse.org/
• Did you get any type checking errors from Babel Typecheck

Optional

• If it's a new component, is it in ES6? Is it clear of warnings from ESLint?
• Have you replaced any ChangeListener or PromiseRenderer components with code that updates component state?
• If changes are made to the classifier, does the dev classifier still work?
• Have you added in flow type annotations?

[camallen]

LGTM

[eatyourgreens]

Leaving the branch here as a reminder until changes have been copied to zooniverse/markdownz#58

[jelliotartz]

Thanks @eatyourgreens, changes copied into zooniverse/markdownz#58, closing this branch now.