Initial FAQ, with hopefully more contributions in the future. (#1202)

docs/FAQ.md

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+# Frequently Asked Questions (FAQ)
+
+## Stream Modality
+
+Questions related to stream modality (e.g., [MultiStreamTracker][multi-stream-tracker]).
+
+### What is the impact of transitioning my KCL app from single-stream to multi-stream?
+
+This answer assumes the [StreamTracker][stream-tracker] implementation is being changed.
+From KCL's perspective, there is no modality change by decreasing a multi-stream tracker from `N` streams to `1` stream.
+
+The DDB `leaseKey`, used to persist metadata including lease checkpoint, has a modality-dependent format:
+
+| Modality | `leaseKey` Format |
+| --- | --- |
+| single-stream | `<shardId>` |
+| multi-stream  | `<accountId>:<streamName>:<streamCreationTimestamp>:<shardId>` |
+
+Transitioning an app -- either from single- to multi-, or vice versa -- creates a backwards-incompatible expectation on the `leaseKey`.
+As a result, a KCL app will be blind to any `leaseKey`, and its checkpoint, that does not match the expected format.
+
+## Resources
+
+For additional information, please consider reading:
+* https://docs.aws.amazon.com/streams/latest/dev/kcl-migration.html
+* https://docs.aws.amazon.com/streams/latest/dev/shared-throughput-kcl-consumers.html
+
+[multi-stream-tracker]: /amazon-kinesis-client/src/main/java/software/amazon/kinesis/processor/MultiStreamTracker.java
+[stream-tracker]: /amazon-kinesis-client/src/main/java/software/amazon/kinesis/processor/StreamTracker.java