docs(samples): Add an example of using read_time in queries and get()

[jlara310]

Adds an example of how to use read_time (added in #334).

Fixes #341 🦕

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/snippets.py:61, tag datastore_snapshot_read

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[jlara310]

@yixiaoshen , please review.