add compilation support

* document SPC C
* SPC R for recompile
* helm-make-projectile

Oleh was quick on adding the helm-make-projectile feature. Thanks! https://github.com/abo-abo/helm-make/issues/3
@rphillips What do we do about `<SPC> R` which is already taken by [pcre2el](https://github.com/syl20bnr/spacemacs/tree/master/contrib/pcre2el#keybindings) ?