document how helpers can be used in lean strings

[fuzetsu]

I've been using bss for a while now, but I had no idea that I could use helpers inside of lean strings.

Based on the documentation I thought they were only accessible in the following way:

m('div' + b.f1.p1.size(200).align('center'))

Recently I discovered this is valid as well:

m('div' + b`f1;p1;size 200;align center`)

I only discovered this because @osban posted this flems on the mithril gitter.

[porsager]

Oh yeah, that'd be really great 😊 I'm very open to PRs 😉