Add 'Sending email with attachment' Use Case

[ittus]

Add document for #270

Need to use base64.b64encode(data).decode() because in python3, b64encode returns bytes instead of string

Reference:
https://docs.python.org/2/library/base64.html
https://docs.python.org/3/library/base64.html

[thinkingserious]

Hello @ittus,

Thank you for the PR!

We will be reviewing this PR soon, in the mean time, if you have not, please be sure to sign our CLA so that we can merge your changes. Thanks again!

Team DX

[andriisoldatenko]

https://github.com/elbuo8/sendgrid-django/blob/master/sgbackend/mail.py#L110-L118
👍

[thinkingserious]

Thanks again for the PR @ittus!

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