Update FAQ with key export instructions

Signed-off-by: Stanislav Bogatyrev <stanislav@nspcc.ru>

docs/faq.md

@@ -1,5 +1,42 @@
 # F.A.Q, tips and tricks
 
+
+### How to export private key from Neo wallet for NeoFS use?
+
+Private key for usage with NeoFS tools can be extracted from Neo wallet in three
+simple steps.
+
+1. Get the key in WIF format
+
+```
+$ docker exec -it main_chain neo-go wallet export -w wallets/wallet.json -d NTrezR3C4X8aMLVg7vozt5wguyNfFhwuFx
+Enter password >
+KxDgvEKzgSBPPfuVfw67oPQBSjidEiqTHURKSDL1R7yGaGYAeYnr
+```
+
+2. Convert form WIF to HEX
+
+```
+$ neofs-cli util keyer KxDgvEKzgSBPPfuVfw67oPQBSjidEiqTHURKSDL1R7yGaGYAeYnr
+PrivateKey      1dd37fba80fec4e6a6f13fd708d8dcb3b29def768017052f6c930fa1c5d90bbb
+PublicKey       031a6c6fbbdf02ca351745fa86b9ba5a9452d785ac4f7fc2b7548ca2a46c4fcf4a
+WIF             KxDgvEKzgSBPPfuVfw67oPQBSjidEiqTHURKSDL1R7yGaGYAeYnr
+Wallet3.0       NTrezR3C4X8aMLVg7vozt5wguyNfFhwuFx
+ScriptHash3.0   12b97a2206ae4b10c7e0194b7b655c32cc912057
+ScriptHash3.0BE 572091cc325c657b4b19e0c7104bae06227ab912
+```
+
+3. Dump into file in binary format
+
+```
+$ echo '1dd37fba80fec4e6a6f13fd708d8dcb3b29def768017052f6c930fa1c5d90bbb' | xxd -r -p > wallets/wallet.key
+$ xxd wallets/wallet.key
+00000000: 1dd3 7fba 80fe c4e6 a6f1 3fd7 08d8 dcb3  ..........?.....
+00000010: b29d ef76 8017 052f 6c93 0fa1 c5d9 0bbb  ...v.../l.......
+```
+
+Later this functionality will be included in `neofs-cli` directly.
+
 ### How to see what's inside running container if there's no shell?
 
 You can run any program in container's namespace using `nsenter` utility.