Add note about Clear-Site-Data header

[righettod]

Hi,

This PR add a reco about where to set the header Clear-Site-Data.

I discovered this thanks to a friend that faced an loading time issue by setting the header on the welcome page of its web app 😉

Thanks in advance 😀.

[righettod]

@riramar Can you merge it if it is OK for you ?

Thanks in advance 😃

[righettod]

Thanks 😃

[riramar]

You're welcome! :)