Extend documentation of "Build requirements"

[SunBlack]

Currently the Build requirements just contains the hint:

• libtiff >= 4.0 (optional but recommended)
• curl >= 7.29.0 (optional but recommended)

There is no hint why they are recommend. So maybe a hint like:

> * libtiff >= 4.0 (optional but recommended to enable the ability to read some grids)
> * curl >= 7.29.0 (optional but recommended to support downloading the required grids during runtime)

Furthermore there is missing a hint, which options are required for libtiff (I assume WebP, zStd, ... are not required be the grids) for the current grids.