Document RuboCop's Style/Lambda should be disabled

[valscion]

I noticed that Style/Lambda cop from RuboCop has a chance on conflicting with the way @prettier/plugin-ruby wants to format code.

Say we have this original code:

something = -> { a_very_long_line_that_needs_to_be_splitted_to_next_one_aaa_bbb }

After prettifying, the code will turn to this:

something = -> do
  a_very_long_line_that_needs_to_be_splitted_to_next_one_aaa_bbb
end

However, RuboCop does not like this, and would prefer multiline lambdas to instead look like this:

something = lambda do
  a_very_long_line_that_needs_to_be_splitted_to_next_one_aaa_bbb
end

But if I did that change, then after prettier run, the code would turn to this one:

something =
  lambda { a_very_long_line_that_needs_to_be_splitted_to_next_one_aaa_bbb }

which Style/Lambda cop would then complain that the code should be like this instead:

something =
  -> { a_very_long_line_that_needs_to_be_splitted_to_next_one_aaa_bbb }

...but if I did that, then we'd be back at this code after prettier run:

something = -> do
  a_very_long_line_that_needs_to_be_splitted_to_next_one_aaa_bbb
end

---

So in the end, what we decided to do was disable Style/Lambda for the Ruby files that are formatted with Prettier. I figured it would be worth it to document this cop needs to be disabled in order not to cause conflicts between RuboCop and @prettier/plugin-ruby.

[kddnewton]

You even did the CHANGELOG entry! Thank you!!