add more structure to dev setup instructions
add more structure to dev setup instructions

Reviewed at https://rbcommons.com/s/twitter/r/1334/

pantsbuild/intellij-pants-plugin,jsirois/intellij-pants-plugin,wisechengyi/intellij-pants-plugin,wisechengyi/intellij-pants-plugin,wisechengyi/intellij-pants-plugin,pantsbuild/intellij-pants-plugin,wisechengyi/intellij-pants-plugin,jsirois/intellij-pants-plugin,pantsbuild/intellij-pants-plugin,jsirois/intellij-pants-plugin,pantsbuild/intellij-pants-plugin,jsirois/intellij-pants-plugin,pantsbuild/intellij-pants-plugin