Header Only needs cpp_info.includedirs

Hello,
I followed the tutorial for a [header only conan package](https://docs.conan.io/en/latest/howtos/header_only.html). For test purpose, I create an little example and include the header only libary with conan. The example is build with cmake. My lib is included via find_package.

I got stuck because no headers are found. After some investigations I found that inside the FindMyLib.cmake the include directory are empty:
`set(MyLib_INCLUDE_DIRS)`
To resolve it I need to add cpp_info.includedirs to the package_info in the header only conan package:
```
def package_info(self):
self.cpp_info.libs = ["MyLib"]
self.cpp_info.includedirs = ['include']
```

The setting of the includedirs should be part of the tutorial.