docs: update custom bubbles requirements (#495)

README.md

@@ -223,8 +223,14 @@ func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
   list that is scrollable without pagination and has the ability to contain
   other bubbles as list items.
 
-If you’ve built a Bubble you think should be listed here,
-[let us know](mailto:[email protected]).
+If you’ve built a Bubble you think should be listed here, please create a Pull
+Request. Please note that for a project to be included, it must meet the
+following requirements:
+- The README has a demo GIF.
+- The README clearly states the purpose of the bubble along with an example on how to use it. 
+- The bubble must *always* be in a working state on its `main` branch.
+
+Thank you!
 
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