Adding Gulp watch documentation

[Dan503]

This PR contains:

• [ ] bugfix
• [ ] feature
• [ ] refactor
• [x] documentation
• [ ] other

Are tests included?

• [ ] yes (bugfixes and features will not be merged without tests)
• [x] no

Breaking Changes?

• [ ] yes (breaking changes will not be merged unless absolutely necessary)
• [x] no

List any relevant issue numbers: https://github.com/rollup/stream/issues/1

Description

This pull request adds documentation to guide users on how to most effectively use @rollup/stream in a Gulp watch task.

I also removed import terser from 'gulp-terser'; from the source-maps example since it wasn't being used in the example and it might confuse people.

I also converted the rollup-stream readme link to a cleaner text based link rather than a url.

[shellscape]

Sorry it took a while to get eyes on this. We've been full steam ahead on the plugins repo and I had admittedly forgotten to watch this repo.