ignore.md: add link to gitignore docs (#6168)
ignore.md: add link to gitignore docs (#6168)

Prettier uses the `ignore` package, which implements gitignore syntax.
So add a link to gitignore syntax.
existentialism/prettier,existentialism/prettier,rattrayalex/prettier,rattrayalex/prettier,existentialism/prettier,existentialism/prettier,prettier/prettier,prettier/prettier,prettier/prettier,jlongster/prettier,rattrayalex/prettier,prettier/prettier