SPONSORS: describe the basics

Closes #13119

docs/Makefile.am

@@ -89,6 +89,7 @@ EXTRA_DIST =                                    \
  RUSTLS.md                                      \
  ROADMAP.md                                     \
  SECURITY-ADVISORY.md                           \
+ SPONSORS.md                                    \
  SSL-PROBLEMS.md                                \
  SSLCERTS.md                                    \
  THANKS                                         \

docs/SPONSORS.md

@@ -0,0 +1,41 @@
+# curl sponsors
+
+A sponsor is someone who donates money or resources to the curl project for no
+specific service in return.
+
+curl accepts donations via [GitHub sponsors](https://github.com/sponsors/curl)
+and [Open Collective](https://opencollective.com/curl).
+
+An even better way to contribute to the project might be to pay an engineer or
+two to spend work hours on curl related tasks.
+
+We promise to use donated funds for things and activities that we believe are
+beneficial for the project and its development. That includes but is not
+limited to bug-bounties, developer conferences, infrastructure, development,
+services and hardware.
+
+Recurring donations above a certain amount of money puts the sponsor at a
+named sponsor level: **Silver**, **Gold**, **Platinum** or **Top**.
+
+Sponsors on a named level can provide their logo image and preferred URL and
+get recognition on the curl website's [sponsor
+page](https://curl.se/sponsors.html).
+
+- **Silver Sponsor** at least 100 USD/month
+- **Gold Sponsor** at least 500 USD/month
+- **Platinum Sponsor** at least 1000 USD/month
+- **Top Sponsor** outstanding extra valuable help
+
+## Sponsor requirements
+
+A named level sponsor is entitled a logo and link on the curl website assuming
+the company, brand and link are not deemed unsuitable. The curl team reserves
+the right to make that decision at its own discretion.
+
+Sponsors may be denied a website presence for example if involved with drugs,
+gambling, pornography, social media manipulation etc.
+
+## Past Sponsors
+
+Sponsors that stop paying are considered *Past Sponsors* and are not displayed
+on the sponsor page anymore. We thank you for your contributions!