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
Already taken by pcre2el layer. We put these bindings behind prefix `<SPC> C` leading to <SPC> C c` and `<SPC> C r` (or `<SPC> C C` ?) but I wonder if this bindings are too complicated for pretty frequent commands.