Simple how-to tutorial walking through upgrading a TL-WDR3600 router with stock firmware to an OpenWRT 12 firmware. For a more recent example and better hardware recommendation please see • Installing OpenWRT on WRT1200AC for installing OpenWRT 15 on a WRT1200AC.
2018 UPDATE: TPLink has locked the router firmware and does not allow OpenWRT or LEDE installations on later versions of this router hardware.