Fix/readme

[DaanOolbekkink]

Proposed changes

Small adjustment to the readme. The request Request.addCookie() should be awaited and that is not specified in the readme

Type of changes

What types of changes does your code introduce?

Put an x in the boxes that apply.

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• My code follows the Dart coding convention
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works.
  • Tests show the use of await, hence the PR
• I have commented my code, particularly in hard-to-understand areas
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change,
kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...