Add `make switch` fore easy installation of dependencies

[aqjune]

This patch adds make switch that

• Creates a local OPAM switch under the current hol-light directory
• Chooses the latest fully supported OCaml version (4.14 for now; would be great if it is 5.0 in the future!)
• And installs dependencies that are needed by HOL Light.

This is to help beginners set the environment for HOL Light.

I also updated README to explain(recommend) this option. A separate 'DEPENDENCIES' chapter is added to explain the original, detailed steps.