Document building the plugin

[jasalt]

To avoid startup cost, figure out how building works in this context.

https://phel-lang.org/documentation/cli-commands/#build-the-project

[jasalt]

New caching mechanism in https://github.com/phel-lang/phel-lang/releases/tag/v0.27.0 seems to help with the Phel's (1-2 sec) cold startup time so that manually transpiling/compiling to PHP ahead of time shouldn't be that necessary anymore to have fast page load speeds.