Add initial limits doc

doc/administration/instance_limits.md

+---
+type: reference
+---
+
+# GitLab application limits
+
+GitLab, like most large applications, enforces limits within certain features to maintain a
+minimum quality of performance. Allowing some features to be limitless could affect security,
+performance, data, or could even exhaust the allocated resources for the application.
+
+## Number of comments per issue, merge request or commit
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/22388) in GitLab 12.4.
+
+There's a limit to the number of comments that can be submitted on an issue,
+merge request, or commit. When the limit is reached, system notes can still be
+added so that the history of events is not lost, but user-submitted comments
+will fail.
+
+- **Max limit:** 5.000 comments