Improve README as beginners' guide

[ybiquitous]

Hi, there.

When I read README, I spent time to know what the most recommended way to use gem_rbs_collection is.

For example, the README describes the easiest way is to use the git submodule command, but I understand the most recommended way is to use the rbs collection command today, as this @pocke's article explains.

And, since the current README doesn't refer to TypeProf, I feel a bit unhelpful for TypeProf users.

So, I suggest:

• adds how to use gem_rbs_collection with rbs collection first
  • it may be just good to add a simple description and a link to the rbs collection doc
• adds the usage with TypeProf
• sort each usage in order of frequency or importance, e.g.
  • rbs collection (standard)
  • TypeProf (standard)
  • Steep (mostly used)
  • git submodule
  • etc.

What do you think about the suggestion? Welcome to any feedback! I could also rewrite the README if you would hope.

Thanks.