Add in-tree mspec and ruby/spec

* For easier modifications of ruby/spec by MRI developers.
* .gitignore: track changes under spec.
* spec/mspec, spec/rubyspec: add in-tree mspec and ruby/spec.
  These files can therefore be updated like any other file in MRI.
  Instructions are provided in spec/README.
  [Feature #13156] [ruby-core:79246]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58595 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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diff --git a/spec/rubyspec/CONTRIBUTING.md b/spec/rubyspec/CONTRIBUTING.md
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+Contributions are much appreciated.
+Please open a pull request or add an issue to discuss what you intend to work on.
+If the pull requests passes the CI and conforms to the existing style of specs, it will be merged.
+
+### File organization
+
+Spec are grouped in 5 separate top-level groups:
+
+* `command_line`: for the ruby executable command-line flags (`-v`, `-e`, etc)
+* `language`: for the language keywords and syntax constructs (`if`, `def`, `A::B`, etc)
+* `core`: for the core methods (`Fixnum#+`, `String#upcase`, no need to require anything)
+* `library`: for the standard libraries methods (`CSV.new`, `YAML.parse`, need to require the stdlib)
+* `optional/capi`: for functions available to the Ruby C-extension API
+
+The exact file for methods is decided by the `#owner` of a method, for instance for `#group_by`:
+```ruby
+> [].method(:group_by)
+=> #<Method: Array(Enumerable)#group_by>
+> [].method(:group_by).owner
+=> Enumerable
+```
+Which should therefore be specified in `core/enumerable/group_by_spec.rb`.
+
+### MkSpec - a tool to generate the spec structure
+
+If you want to create new specs, you should use `mkspec`, part of [MSpec](http://github.com/ruby/mspec).
+
+    $ ../mspec/bin/mkspec -h
+
+#### Creating files for unspecified modules or classes
+
+For instance, to create specs for `forwardable`:
+
+    $ ../mspec/bin/mkspec -b library -rforwardable -c Forwardable
+
+Specify `core` or `library` as the `base`.
+
+#### Finding unspecified core methods
+
+This is very easy, just run the command below in your `spec` directory.
+`ruby` must be a recent version of MRI.
+
+    $ ruby --disable-gem ../mspec/bin/mkspec
+
+You might also want to search for:
+
+    it "needs to be reviewed for spec completeness"
+
+which indicates the file was generated but the method unspecified.
+
+### Guards
+
+Different guards are available as defined by mspec.
+In general, the usage of guards should be minimized as possible.
+
+There are no guards to define implementation-specific behavior because
+the Ruby Spec Suite defines common behavior and not implementation details.
+Use the implementation test suite for these.
+
+If an implementation does not support some feature, simply tag the related specs as failing instead.
+
+### Style
+
+Do not leave any trailing space and respect the existing style.