Add Slim templates about torch

[yordis]

@danielberkompas I would leave the project embracing EEx only mate. I wouldn't split the community between EEx and Slim as it happened in the Rails ecosystem.

Realistically from the experience of Ruby ecosystem, it just add friction and without techinical reasons to back it up outside of personal preferences.

But, that is just my opinion.

[chevinbrown]

For me, adding templating other than the defaults chosen by phoenix only hurt the core application.
I ❤️ slim, but for other members on my team I could see this causing adoption friction. Just an outsider's 2 cents. 😸

[jivko-chobanov]

Please mention that in the docs front page. Maybe around: "NOTE: If you choose to use slim templates, you will need to install Phoenix Slim.".

I lost several hours trying to debug torch, expecting it to have slim support as it proclaims in the docs.

[cpjolicoeur]

This is now noted in the README. Closing for now.