[#97] Document notary service in chains

`notary.md` contains brief description about
notary service and steps to enable or disable
it in neofs-dev-env chains.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

docs/notary.md

@@ -0,0 +1,81 @@
+# Notary service in chains
+
+[Notary service](https://github.com/neo-project/neo/issues/1573#issuecomment-704874472)
+is a service that provides on-chain network assistance to form multisignature 
+transactions. Inner Ring (Alphabet) nodes use multisignature transactions to
+create containers, approve balance changes, update network map, tick epochs, 
+etc. With notary service, it takes up to seven times fewer transactions 
+to do these operations. Notary service calculates the exact amount of GAS 
+to execute transaction, therefore operations are cheaper (withdraw fee **with**
+notary is less than 0.5 GAS; withdraw fee **without** notary is up to 7.0 GAS). 
+
+By default, main chain service is running without notary service, and side chain
+running with notary service. However, you can change that in configuration.
+
+# Disable notary service in side chain
+
+To disable notary service in side chain do these steps.
+
+1. Update `.env` and choose notary disabled chain dump for side chain.
+
+```
+MORPH_CHAIN_URL="https://github.com/nspcc-dev/neofs-contract/releases/download/v0.9.0/devenv_sidechain_notary_disabled.gz"
+```
+
+Make sure to update chain dump files with `make get` target.
+
+2. Update `service/morph_chain/protocol.privnet.yml` and disable notary settings
+and state root in header.
+   
+```yaml
+ProtocolConfiguration:
+  StateRootInHeader: false
+  P2PSigExtensions: false
+ApplicationConfiguration:
+  P2PNotary:
+    Enabled: false
+```
+
+3. Update `services/ir/.ir.env` and disable notary support.
+```
+NEOFS_IR_WITHOUT_NOTARY=true
+```
+
+Chain dump without notary service does not have predefined network map.
+Therefore, you need to wait about 5 minutes until new epoch tick with updated
+network map.
+
+All helper commands such as `make tick.epoch` or `make update.epoch_duration`
+are designed as multisignature transactions, so you can't use them without
+notary.
+
+# Enable notary service in main chain
+
+To enable notary service in main chain do these steps.
+
+1. Update `.env` and choose notary enabled chain dump for main chain.
+
+```
+CHAIN_URL="https://github.com/nspcc-dev/neofs-contract/releases/download/v0.9.0/devenv_mainchain.gz"
+```
+
+Make sure to update chain dump files with `make get` target.
+
+2. Update `service/chain/protocol.privnet.yml` and enable notary settings.
+
+```yaml
+ProtocolConfiguration:
+  P2PSigExtensions: true
+ApplicationConfiguration:
+  P2PNotary:
+    Enabled: true
+```
+
+3. Update `services/ir/.ir.env` and enable main chain notary support.
+```
+NEOFS_IR_WITHOUT_MAIN_NOTARY=false
+```
+
+Main chain generates a block once per 15 seconds, so Inner Ring takes about 
+15-30 seconds to make a notary deposit in main chain after startup. Then 
+neofs-dev-env is ready to work.