Add new next steps page to the documentation

docs/installation/10_next_steps.md

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+---
+title: Next steps
+parent: Installation
+has_toc: true
+nav_order: 10
+permalink: /installation/start/nextsteps
+---
+
+# Next steps
+
+Now that you have Dendrite running, the following tweaks will improve the reliability
+of your installation.
+
+## File descriptor limit
+
+Most platforms have a limit on how many file descriptors a single process can open. All
+connections made by Dendrite consume file descriptors — this includes database connections
+and network requests to remote homeservers. When participating in large federated rooms
+where Dendrite must talk to many remote servers, it is often very easy to exhaust default
+limits which are quite low.
+
+We currently recommend setting the file descriptor limit to 65535 to avoid such
+issues. Dendrite will log immediately after startup if the file descriptor limit is too low:
+
+```
+level=warning msg="IMPORTANT: Process file descriptor limit is currently 1024, it is recommended to raise the limit for Dendrite to at least 65535 to avoid issues"
+```
+
+UNIX systems have two limits: a hard limit and a soft limit. You can view the soft limit
+by running `ulimit -Sn` and the hard limit with `ulimit -Hn`:
+
+```
+$ ulimit -Hn
+1048576
+
+$ ulimit -Sn
+1024
+```
+
+Increase the soft limit before starting Dendrite:
+
+```
+ulimit -Sn 65535
+```
+
+The log line at startup should no longer appear if the limit is sufficient.
+
+If you are running under a systemd service, you can instead add `LimitNOFILE=65535` option
+to the `[Service]` section of your service unit file.
+
+## DNS caching
+
+Dendrite has a built-in DNS cache which significantly reduces the load that Dendrite will
+place on your DNS resolver. This may also speed up outbound federation.
+
+Consider enabling the DNS cache by modifying the `global` section of your configuration file:
+
+```
+  dns_cache:
+    enabled: true
+    cache_size: 4096
+    cache_lifetime: 600s
+```
+
+## Time synchronisation
+
+Matrix relies heavily on TLS which requires the system time to be correct. If the clock
+drifts then you may find that federation no works reliably (or at all) and clients may
+struggle to connect to your Dendrite server.
+
+Ensure that the time is synchronised on your system by enabling NTP sync.