Here is a good link for rooting the Huawei P9 phone.
Unfortunately that article points to the wrong firmware, which is required for my phone (Huawei P9 Lite). This can be found here. It is crucial to have the exact firmware to flash, otherwise there is the risk of bricking the phone for good.
Also, in order to register on Huawei page to get the oem code required for unlocking the bootloader, use this link for english speakers.
Finally a good intro on android rooting can be found here.