Example Airflow DAG that sends an email message through SendGrid

[pavel-salnikov]

Description

Example Airflow DAG that sends an email message through SendGrid.

Checklist

• [X] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [X] README is updated to include all relevant information
• [ ] Tests pass: nox -s py-3.6 (see Test Environment Setup)
• [ ] Lint pass: nox -s lint (see Test Environment Setup)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [X] Please merge this PR for me once it is approved.
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample

[snippet-bot[bot]]

Here is the summary of changes.

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• composer/workflows/email_operator_sendgrid.py:24, tag composer_email_operator_sendgrid

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