Added doc for external feegrant config

docs/advanced_usage.md

@@ -68,6 +68,10 @@ For example, configure feegrants for Kujira:
 - Note: above, `default` is the key that will need to contain funds (the granter)
 - 10 grantees will be configured, so those 10 address will sign TXs in round robin order.
 
+An external feegrant configuration can be applied with the following command:
+- `rly chains configure feegrant basicallowance cosmoshub cosmosaddr --grantees grantee3`
+- Note: above, `cosmosaddr` is a bech32 address that has already issued a feegrant allowance to `grantee3`.
+- External configuration means that someone else controls `cosmosaddr` (you do not need the mnemonic).
 
 You may also choose to specify the exact names of your grantees:
 - `rly chains configure feegrant basicallowance kujira default --grantees "kuji1,kuji2,kuji3"`
@@ -78,7 +82,6 @@ Rerunning the feegrant command will simply confirm your configuration is correct
 To remove the feegrant configuration:
 - `rly chains configure feegrant basicallowance kujira --delete`
 
-
 ## Stuck Packet
 
 There can be scenarios where a standard flush fails to clear a packet due to differences in the way packets are observed. The standard flush depends on the packet queries working properly. Sometimes the packet queries can miss things that the block scanning performed by the relayer during standard operation wouldn't. For packets affected by this, if they were emitted in recent blocks, the `--block-history` flag can be used to have the standard relayer block scanning start at a block height that many blocks behind the current chain tip. However, if the stuck packet occurred at an old height, farther back than would be reasonable for the `--block-history` scan from historical to current, there is an additional set of flags that can be used to zoom in on the block heights where the stuck packet occurred.