Adding Accumulator Example

[joshmsmith]

What was changed

Adding accumulator pattern sample: accumulator and associated readme changes

Why?

This is a pattern that's pretty common, and I wanted to have a sample that I could share about it. Gathering a lot of signals, managing continue as new, demonstrating signal with start, all good stuff to have.

Checklist

1. Closes N/A

2. How was this tested: Tested locally, tests in accumulate_signals_workflow_test.go

3. Any docs updates needed? No

[cretz]

@Quinn-With-Two-Ns - unsure if you want to give this a review

[Quinn-With-Two-Ns]

@cretz I saw this, I just had no additional comments to add after yours