message: Updated snapshot tests README to include information about what these tests do.
diff: # @fluid-internal/test-snapshots
  +The snapshot tests primarily validate compatibility of Fluidframework with older snapshot format, i.e., snapshots that were generated by older versions of Fluidframework runtime work correctly with the current version. Examples of some of the tests:
  +- Load older snapshots with current version of container and validate the load is successful.
  +- Load older snapshots with current version of container, take new snapshots and validate the snapshots are in the current snapshot format.
  +- Messages generated by an older container are replayed to a container running current version. At some intervals during replay, snapshots are taken and validated that they are correct.
  +
  +A list of old snapshots are stored in [FluidFrameworkTestData](https://github.com/microsoft/FluidFrameworkTestData) repo which is a submodule of Fluidframework repo under the `content` folder. The sub-folders under `snapshotTestContent` represent snapshots generated by a container with content that helps validate compatibility of different functionalities.
  +For example, `SharedMatrixDeletes` has snapshots that where delete operations were performed on the `SharedMatrix` DDS and it validates the compatibility of this functionality.
  +
  +For each of these sub-folders, snapshots with older versions are stored under `src_snapshots` and snapshots with current (or latest) version are stored under `current_snapshots`.
  +
  +Note that the snapshots under `src_snapshots` are stored under folders whose name represent the runtime version that generated these snapshots. Every time the snapshot format changes, the current snapshot are moved to `src_snapshots` under the current runtime version number and the new snapshots are stored in `current_snapshots`.
  +
  +For example, snapshots under `/SharedMatrixDeletes/src_snapshots/0.58.1000` were generated by runtime version `0.58.1000`. Now, lets say that the snapshot format changes in runtime version `X.0.0`. The snapshots under `/SharedMatrixDeletes/current_snapshots` will move to `/SharedMatrixDeletes/src_snapshots/X.0.0` and the new snapshots will be stored in the `current_snapshots` folder.
  +
  ## Syncing snapshot tests content
  -If it is the first time you are syncing the content for the snapshot tests, you need to run the following command:
  +The snapshots are stored in the [FluidFrameworkTestData](https://github.com/microsoft/FluidFrameworkTestData) which is a submodule of Fluidframework repo. Run the following command to download / sync the snapshots:
  ```
  git submodule update --init --recursive
  