Add basecamp third-party provider (#830)

Co-authored-by: Ben Ramsey <[email protected]>

Author: stevenmaguire

docs/providers/thirdparty.md

@@ -26,6 +26,7 @@ Gateway | Composer Package | Maintainer
 [Auth0](https://github.com/RiskioFr/oauth2-auth0) | riskio/oauth2-auth0 | [Nicolas Eeckeloo](https://github.com/neeckeloo)
 [Azure Active Directory](https://github.com/thenetworg/oauth2-azure) | thenetworg/oauth2-azure | [Jan Hajek](https://github.com/hajekj)
 [BASE](https://github.com/w-takumi/oauth2-base) | shippinno/oauth-base | [SHIPPInno Corp.](https://www.shippinno.co.jp/)
+[Basecamp](https://github.com/stevenmaguire/oauth2-basecamp) | stevenmaguire/oauth2-basecamp | [Steven Maguire](https://github.com/stevenmaguire)
 [Battle.net](https://github.com/tpavlek/oauth2-bnet) | depotwarehouse/oauth2-bnet | [Troy Pavlek](https://github.com/tpavlek)
 [Bitbucket](https://github.com/stevenmaguire/oauth2-bitbucket) | stevenmaguire/oauth2-bitbucket | [Steven Maguire](https://github.com/stevenmaguire)
 [BookingSync](https://github.com/BookingSync/oauth2-bookingsync-php) | bookingsync/oauth2-bookingsync-php | [BookingSync](https://github.com/BookingSync)