Point corporate donors toward our OC contact form

A corporate donor recently wasn't able to find a clear way to contact a Homebrew person to talk about a possible donation. They eventually found the OC contact us page, which is the best way to contact the PLC short without having a GitHub account and posting on an issue or Discussions.

Let's highlight the donations questions contact form specifically in this area that is the landing target for the donations link on https://brew.sh.

README.md

@@ -27,6 +27,8 @@ Homebrew is a non-profit project run entirely by unpaid volunteers. We need your
 
 Please consider a regular donation through [GitHub Sponsors](https://github.com/sponsors/Homebrew), [Open Collective](https://opencollective.com/homebrew) or [Patreon](https://www.patreon.com/homebrew). Homebrew is fiscally hosted by the [Open Source Collective](https://opencollective.com/opensource).
 
+For questions about donations, including corporate giving, please contact us through our Open Collective [contact form](https://opencollective.com/homebrew/contact).
+
 ## Community
 
 - [Homebrew/discussions (forum)](https://github.com/orgs/Homebrew/discussions)