Add link to bot in projects using all contributors

[jakebolam]

Describe the solution you'd like
We want to gain more widespread adoption. An easy way to do this is have project's link back/credit us when using the bot. We could add a link under the all contributors table to the bot.

Something like

Table created by the all contributors bot

Or

Generated by the all contributors bot, check out the usage docs to add your contribution.

[Berkmann18]

Have you started working on that?

[tenshiAMD]

Resolved in #163.