Add documentation for update_url parameters

[mkaply]

We should document the various parameters that can be passed to an update_url here:

https://extensionworkshop.com/documentation/manage/updating-your-extension/

I believe all these get substituted even on a non AMO update URL:

%REQ_VERSION%
%ITEM_ID%
%ITEM_VERSION%
%ITEM_MAXAPPVERSION%
%ITEM_STATUS%
%APP_ID%
%APP_VERSION%
%APP_OS%
%APP_ABI%
%APP_LOCALE%
%CURRENT_APP_VERSION%
%UPDATE_TYPE%
%COMPATIBILITY_MODE%