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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+# The Ruby Spec Suite
+
+[![Build Status](https://travis-ci.org/ruby/spec.svg)](https://travis-ci.org/ruby/spec)
+[![Build Status](https://ci.appveyor.com/api/projects/status/1gs6f399320o44b1?svg=true)](https://ci.appveyor.com/project/eregon/spec-x948i)
+
+The Ruby Spec Suite is a test suite for the behavior of the Ruby programming language.
+
+It is not a standardized specification like the ISO one, and does not aim to become one.
+Instead, it is a practical tool to describe and test the behavior of Ruby with code.
+
+Every example code has a textual description, which presents several advantages:
+
+* It is easier to understand the intent of the author
+* It documents how recent versions of Ruby should behave
+* It helps Ruby implementations to agree on a common behavior
+
+The specs are written with syntax similar to RSpec 2.
+They are run with MSpec, the purpose-built framework for running the Ruby Spec Suite.
+For more information, see the [MSpec](http://github.com/ruby/mspec) project.
+
+The specs describe the [language syntax](language/), the [core library](core/), the [standard library](library/), the [C API for extensions](optional/capi) and the [command line flags](command_line/).
+The language specs are grouped by keyword while the core and standard library specs are grouped by class and method.
+
+ruby/spec is known to be tested in these implementations for every commit:
+* [MRI](http://rubyci.org/) on 30 platforms and 4 versions
+* [JRuby](https://github.com/jruby/jruby/tree/master/spec/ruby) on Travis for both 1.7 and 9.x
+* [TruffleRuby](https://github.com/graalvm/truffleruby) on Travis
+* [Opal](https://github.com/opal/opal/tree/master/spec) on Travis
+
+### Running the specs
+
+First, clone this repository:
+
+    $ git clone https://github.com/ruby/spec.git
+
+Then move to it:
+
+    $ cd spec
+
+Clone [MSpec](http://github.com/ruby/mspec):
+
+    $ git clone https://github.com/ruby/mspec.git ../mspec
+
+And run the spec suite:
+
+    $ ../mspec/bin/mspec
+
+This will execute all the specs using the executable named `ruby` on your current PATH.
+
+### Running Specs with a Specific Ruby Implementation
+
+Use the `-t` option to specify the Ruby implementation with which to run the specs.
+The argument may be a full path to the Ruby binary.
+
+    $ ../mspec/bin/mspec -t /path/to/some/bin/ruby
+
+### Running Selected Specs
+
+To run a single spec file, pass the filename to `mspec`:
+
+    $ ../mspec/bin/mspec core/kernel/kind_of_spec.rb
+
+You can also pass a directory, in which case all specs in that directories will be run:
+
+    $ ../mspec/bin/mspec core/kernel
+
+Finally, you can also run them per group as defined in `default.mspec`.
+The following command will run all language specs:
+
+    $ ../mspec/bin/mspec :language
+
+In similar fashion, the following commands run the respective specs:
+
+    $ ../mspec/bin/mspec :core
+    $ ../mspec/bin/mspec :library
+    $ ../mspec/bin/mspec :capi
+
+### Contributing
+
+See [CONTRIBUTING.md](https://github.com/ruby/spec/blob/master/CONTRIBUTING.md).
+
+### History and RubySpec
+
+This project was originally born from [Rubinius](https://github.com/rubinius/rubinius) tests being converted to the spec style.
+These specs were later extracted to their own project, RubySpec, with a specific vision and principles.
+At the end of 2014, Brian Shirai, the creator of RubySpec, decided to [end RubySpec](http://rubinius.com/2014/12/31/matz-s-ruby-developers-don-t-use-rubyspec/).
+A couple months later, the different repositories were merged and [the project was revived](http://eregon.github.io/rubyspec/2015/07/29/rubyspec-is-reborn.html).
+On 12 January 2016, the name was changed to "The Ruby Spec Suite" for clarity and to let the RubySpec ideology rest in peace.