Add an ADR about the Auditable type and helper fn

docs/architecture/decision-records/Auditable.md

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+# Auditable
+
+## Context
+
+A historical record needs to be kept related to certain data points in CAMS. This historical record involves tracking when a change was made, and by whom. We were previously managing this for orders and assignments in an ad hoc manner. To ensure consistency and to reduce boilerplate code, we need a way to provide this functionality through code common to all data types that require historical records.
+
+## Decision
+
+We have created the `Auditable` type which contains the common properties for history. We have also created the `createAuditRecord` function which provides default values for the properties and the ability to override if needed. Consider the example type—`Foo`—as follows:
+
+```typescript
+type Foo = Auditable & {
+  prop1: string;
+  prop2: number;
+}
+```
+
+To create a historical record for an action initiated by a user, call the `createAuditRecord` as follows:
+
+```typescript
+createAuditRecord<Foo>(someFoo, userSession);
+```
+
+To create a historical record for an action initiated by the system, call the `createAuditRecord` as follows:
+
+```typescript
+createAuditRecord<Foo>(someFoo);
+```
+
+To create a historical record with an override, call the `createAuditRecord` as follows:
+
+```typescript
+const override = { updatedOn: someDate, updatedBy: someUser };
+createAuditRecord<Foo>(someFoo, override);
+```
+
+## Status
+
+Approved
+
+## Consequences
+
+Developers need to remember to make use of this type and the function, but it should reduce the amount of boilerplate/duplicate code we have to write to track history.