Add advance configuration section to readme

[dhoer]

The advanced configuration section in this wonderful article made me realize the power of git-secrets: http://ebi-cloud-portal.readthedocs.io/en/latest/securing_credentials.html#advanced-configuration

I think it would be beneficial to add this information to the readme.

[hyandell]

Can't just copy from someone else's repo/documentation though. Did you get permission or find a license for it?

[dhoer]

They were kind enough to grant us permission.

[hyandell]

Thank you Dennis :)

[hyandell]

@mtdowling This looks good Michael, though I think the scan-history bit is redundant so I would remove that after merge. What do you think?

[mtdowling]

Thanks!