Mention ram limits

We do have that setting in the installation instructions, but we never explain what it's for, so people don't see it / don't copy it.

content/en/docs/install/faqs.md

@@ -4,6 +4,31 @@ weight: 1300
 toc: false
 ---
 
+## My instance keeps crashing, saying "JavaScript heap out of memory", but I have plenty of RAM!
+
+There are 2 problems here:
+
+* `nodesjs` won't use more than a pre-determined amount of RAM (we can
+  change this)
+
+* something causes Sharkey (and almost certainly Misskey) to randomly
+  allocate huge amounts of RAM and die (we would love to fix this, but
+  we haven't been able to figure out the cause, any help is
+  appreciated)
+
+To tell `nodejs` to allocate up to (for example) 8GiB (8192MiB), you
+can set the environmnt variable `NODE_OPTIONS`:
+
+```bash
+export NODE_OPTIONS='--max-old-space-size=8192'
+```
+
+in a startup script, or in your container options, or whatever works
+for your particular setup. The important thing is that the `nodejs`
+process (usually created via the `pnpm start` command) sees the
+variable. Do *not* tell `nodejs` that it can allocate more RAM than
+you have free!
+
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