add docs for append feature to if-docs

[jmcook1186]

What
add documentation explaining the append feature

Why

Context

Prerequisites/resources

SoW (scope of work)

• documentation updated
• test cases added

Acceptance criteria

Given (Setup): Describes the initial state of the system or the preconditions for a change.

When (Action): Describes the specific action or behavior that is being tested/changed.

Then (Assertion): Defines the expected outcome or behavior of the system after the action in the "When" step is performed.