Update Identity pallet README.md (#5183)

Update Identity pallet README.md according to the up-to-date docs,
particularly to explain the _username_ concept of the pallet.

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/frame/identity/README.md

-# Identity Module
+# pallet-identity
 
-- [`identity::Config`](https://docs.rs/pallet-identity/latest/pallet_identity/trait.Config.html)
-- [`Call`](https://docs.rs/pallet-identity/latest/pallet_identity/enum.Call.html)
+## Identity Pallet
 
-## Overview
+- [`Config`]
+- [`Call`]
+
+### Overview
 
 A federated naming system, allowing for multiple registrars to be added from a specified origin.
 Registrars can set a fee to provide identity-verification service. Anyone can put forth a
@@ -23,32 +25,53 @@ by definition, these have equivalent ownership and each has an individual name.
 The number of registrars should be limited, and the deposit made sufficiently large, to ensure
 no state-bloat attack is viable.
 
-## Interface
+#### Usernames
+
+The pallet provides functionality for username authorities to issue usernames. When an account
+receives a username, they get a default instance of `IdentityInfo`. Usernames also serve as a
+reverse lookup from username to account.
+
+Username authorities are given an allocation by governance to prevent state bloat. Usernames
+impose no cost or deposit on the user.
 
-### Dispatchable Functions
+Users can have multiple usernames that map to the same `AccountId`, however one `AccountId` can
+only map to a single username, known as the *primary*.
 
-#### For general users
+### Interface
+
+#### Dispatchable Functions
+
+##### For General Users
 - `set_identity` - Set the associated identity of an account; a small deposit is reserved if not
   already taken.
 - `clear_identity` - Remove an account's associated identity; the deposit is returned.
 - `request_judgement` - Request a judgement from a registrar, paying a fee.
 - `cancel_request` - Cancel the previous request for a judgement.
+- `accept_username` - Accept a username issued by a username authority.
+- `remove_expired_approval` - Remove a username that was issued but never accepted.
+- `set_primary_username` - Set a given username as an account's primary.
+- `remove_dangling_username` - Remove a username that maps to an account without an identity.
 
-#### For general users with sub-identities
+##### For General Users with Sub-Identities
 - `set_subs` - Set the sub-accounts of an identity.
 - `add_sub` - Add a sub-identity to an identity.
 - `remove_sub` - Remove a sub-identity of an identity.
 - `rename_sub` - Rename a sub-identity of an identity.
 - `quit_sub` - Remove a sub-identity of an identity (called by the sub-identity).
 
-#### For registrars
+##### For Registrars
 - `set_fee` - Set the fee required to be paid for a judgement to be given by the registrar.
 - `set_fields` - Set the fields that a registrar cares about in their judgements.
 - `provide_judgement` - Provide a judgement to an identity.
 
-#### For super-users
+##### For Username Authorities
+- `set_username_for` - Set a username for a given account. The account must approve it.
+
+##### For Superusers
 - `add_registrar` - Add a new registrar to the system.
 - `kill_identity` - Forcibly remove the associated identity; the deposit is lost.
+- `add_username_authority` - Add an account with the ability to issue usernames.
+- `remove_username_authority` - Remove an account with the ability to issue usernames.
 
 [`Call`]: ./enum.Call.html
 [`Config`]: ./trait.Config.html