Add examples of limitations to PR template (#1929)

.github/pull_request_template.md

@@ -45,6 +45,9 @@ to submit entries to the PSL as a means to work around those limits or
 restrictions.
 -->
   * [ ] We are listing *any* third-party limits that we seek to work around in our rationale such as those between IOS 14.5+ and Facebook (see [Issue #1245](https://github.com/publicsuffix/list/issues/1245) as a well-documented example)
+    - [Cloudflare](https://developers.cloudflare.com/learning-paths/get-started/add-domain-to-cf/add-site/)
+    - [Letsencrypt](https://letsencrypt.org/docs/rate-limits/)
+    - <UPDATE THIS LIST WITH YOUR LIMITATIONS!>
 
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 The purpose of the question above is to expose limit workarounds.