docs: add a note about open files limit

documentation/en/join-testnet.md

@@ -28,6 +28,8 @@ lotus net peers | wc -l
 
 In order to connect to the network, you need to be connected to at least 1 peer. If you’re seeing 0 peers, read our [troubleshooting notes](https://docs.lotu.sh/en+setup-troubleshooting).
 
+Make sure that you have a reasonable "open files limit" set on your machine, such as 10000. If you're seeing a lower value, such as 256 (default on macOS), read our [troubleshooting notes](https://docs.lotu.sh/en+setup-troubleshooting) on how to update it prior to starting the Lotus daemon.
+
 ## Chain sync
 
 While the daemon is running, the next requirement is to sync the chain. Run the command below to view the chain sync progress. To see current chain height, visit the [network stats page](https://stats.testnet.filecoin.io/).

documentation/en/setup-troubleshooting.md

@@ -29,4 +29,18 @@ ERROR hello hello/hello.go:81 other peer has different genesis!
 - repo is already locked
 ```
 
-- You already have another lotus daemon running.
+- You already have another lotus daemon running.
+
+## Config: Open files limit
+
+On most systems you can check the open files limit with:
+
+```sh
+ulimit -n
+```
+
+You can also modify this number by using the `ulimit` command. It gives you the ability to control the resources available for the shell or process started by it. If the number is below 10000, you can change it with the following command prior to starting the Lotus daemon:
+
+```sh
+ulimit -n 10000
+```