Recommend adding specs folder exclude list in .solargraph.yml

During the testing of the gem with the
[gitlabs](https://gitlab.com/gitlab-org/gitlab/-/blob/master/.solargraph.yml.example?ref_type=heads)
codebase, it resulted that there was a significant slow-down of the
solargraph due to specs pre-parsing at language server boot time.

Turns out, Solargraph treats `exclude:` key in `.solargraph.yml` config
file as "exclude pre-indexing" of directory, which in our case is
exactly what we want, being that specs ought to be isolated from one
another and the only dependency should be actually source code & test
utils.

README.md

@@ -41,7 +41,12 @@ end
 
 If you add them to your Gemfile, you'll have to tell your IDE plugin to use bundler to load the right version of solargraph.
 
-Add `solargraph-rspec` to your `.solargraph.yml` and remove the `spec` directory from the `exclude` list.
+Add `solargraph-rspec` to your `.solargraph.yml` as a plugin.
+
+> [!CAUTION]
+> To avoid **performance issues**, please keep the `spec/**/*` directory in **exclude** list in the Solargraph configuration.
+> That does not actually *exclude* the specs, but rather avoids pre-indexing the specs when Solargraph boots up, and only parses
+> the specs on demand when opened in the editor, which is what we usually want.
 
 (if you don't have a `.solargraph.yml` in your project root, you can run `solargraph config` to add one)
 
@@ -50,7 +55,7 @@ Add `solargraph-rspec` to your `.solargraph.yml` and remove the `spec` directory
  include:
  - "**/*.rb"
  exclude:
--- spec/**/*
++- spec/**/*
  - test/**/*
  - vendor/**/*
  - ".bundle/**/*"