Added glossary explaning Denomination Key (#7330)

When reading this ADR, I was confused about _denomination_. When reading this in context of tokens I always think about a unit / measure. Examples: bills denominated in $20 and $50. Or as 100 cents makes 1 dollar.

The Merriam-Webster dictionary defines [denomination](https://www.merriam-webster.com/dictionary/denomination) as:

1. an act of denominating. Eg: the denomination of prices in U.S. dollars
2. a value or size of a series of values or sizes. Eg: metric denominations
3. name, designation especially : a general name for a category

It seams that we are using the last definition (3rd).

docs/architecture/adr-004-split-denomination-keys.md

@@ -7,6 +7,12 @@
 - 2020-01-14: Updates from review feedback
 - 2020-01-30: Updates from implementation
 
+
+### Glossary
+
+* denom / denomination key -- unique token identifier.
+
+
 ## Context
 
 With permissionless IBC, anyone will be able to send arbitrary denominations to any other account. Currently, all non-zero balances are stored along with the account in an `sdk.Coins` struct, which creates a potential denial-of-service concern, as too many denominations will become expensive to load & store each time the account is modified. See issues [5467](https://github.com/cosmos/cosmos-sdk/issues/5467) and [4982](https://github.com/cosmos/cosmos-sdk/issues/4982) for additional context.