warn about macOS file limit

Summary: The default limit on my macbook air is 256 - that's not enough for clones of larger repos to work.

Reviewed By: DurhamG

Differential Revision: D41308172

fbshipit-source-id: 27972c9f8d66d5236b69f5e65f66eb2e472f52b3

website/docs/introduction/installation.md

@@ -56,6 +56,13 @@ Then install:
 brew install ./{macIntelAsset.name}
 </CodeBlock>
 
+Note that to clone larger repositories, you need to change the open files limit. We recommend doing it now so it doesn't bite you in the future:
+
+<CodeBlock>
+echo "ulimit -n 1048576 1048576" >> ~/.bash_profile{'\n'}
+echo "ulimit -n 1048576 1048576" >> ~/.zshrc
+</CodeBlock>
+
 :::caution
 
 Downloading the bottle using a web browser instead of `curl` will cause macOS to tag Sapling as "untrusted" and the security manager will prevent you from running it. You can remove this annotation as follows: