Add mention of 'Allow less secure apps' to Gmail section of docs

[ethagnawl]

The Gmail example in the README will not work unless "Access for less secure apps" has been turned on from this page. It might be worth adding a note about this to the README, if only to prevent additional issues (like this one) from being opened in the future.

[burhanloey]

Another way is to generate app password if you have two-step verification.