Add imatrix computing tips

As a follow-up to https://huggingface.co/mradermacher/Meta-Llama-3.1-70B-i1-GGUF/discussions/1 and, hopefully, partial redemption for not having located the FAQ before, here's some extra imatrix computing tips that would be nice to add

README.md

@@ -142,6 +142,21 @@ and then run another command which handles download/computation/upload. Most of
 to do stuff when things go wrong (which, with llama.cpp being so buggy and hard to use,
 is unfortunately very frequent).
 
+## What do I need to do to compute imatrix files for large models?
+
+### Hardware
+
+* RAM: A lot of RAM is required to compute imatrix files. Example: 512 GB is just enough to compute 405B imatrix quants in Q8.
+* GPU: At least 8 GB of memory.
+
+### Extra tips
+
+* Computing 405B imatrix quants in Q8 does not seem to have any noticeable quality impact compared to BF16, so to save on hardware
+requirements, use Q8.
+* Sometimes, a single node may not have enough RAM to compute the imatrix file. In such cases, `llama-rpc` inside llama.cpp can
+be used to combine the RAM/VRAM of multiple nodes. This approach takes longer: computing the 405B imatrix file in BF16 takes
+around 20 hours using 3 nodes with 512 GB, 256 GB, and 128 GB of RAM, compared to 4 hours for Q8 on a single node.
+
 ## Why don't you use gguf-split?
 
 TL;DR: I don't have the hardware/resources for that.