More descriptive appExclusions section on README

[stevencaiOR]

Issue: closes #1704

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.

• [✔] issue was opened to discuss proposed changes before starting implementation. It is important do discuss changes before implementing them (Why should we add it? How should it work? How should it look? Where will it be? ...).
• [✔] during development, node version specified in package.json was used (ie using nvm).
• [✔] package versions and package-lock.json were not changed (npm install --no-save).
• [✔] app version number was not changed.
• all new code has tests to ensure against regressions.
• [✔] npm run lint reports no offenses.
• [✔] npm run test is error-free.
• [✔] README and CHANGELOG were updated accordingly.
• [✔] after PR is approved, all commits in it are squashed

Description of the Change

Added a description in README that made adding app exclusions on Windows much clearer. Also made it clear that Stretchly only checks for substrings of running processes for app exclusion rules. No tests because no new code.

Verification Process

No new code. README changes only.