Add Silverstripe sake

[Firesphere]

The New Solution/Problem/Issue/Bug:

Add Silverstripe sake to remove the need to fully type out ddev exec vendor/bin/sake {task}

How this PR Solves The Problem:

With this addition, one can easily install the sake proxy, and run ddev sake {task}

Manual Testing Instructions:

Run the commands described above.

[Firesphere]

I'm sure this is fine, but you might consider creating a DDEV add-on with more facilities than this (and more usable and discoverable). https://ddev.readthedocs.io/en/latest/users/extend/additional-services/

That was indeed a possible next step for me, simply haven't had time, and made this in the interim for my colleagues. Figured might as well PR at :)

[Firesphere]

Just added to the main readme.

I'll have a look at making it a proper additional service later, but for now, a useful start for Silverstripe users, I think

[Firesphere]

Also, a PR would be welcome to the quickstarts

I've written a sort-of quick start for Silverstripe here: https://firesphere.dev/articles/ddevelopment-environment/#e44

I'll see if I can formalize that in to a more useful/complete documentation for the quickstarts!

[rfay]

Cool - the only problem I saw on a quick scan was I always recommend using ddev composer rather than composer on the host, because composer on the host may not match the php configuration (or composer version) of your actual setup inside container.