Initial hacking guide.

Merged: https://github.com/ruby/ruby/pull/5294

doc/hacking.md

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+# Ruby Hacking Guide
+
+This document gives some helpful instructions which should make your experience as a Ruby core developer easier.
+
+## Configure Ruby
+
+It's generally advisable to use a build directory.
+
+	./autogen.sh
+	mkdir build
+	cd build
+	../configure --prefix $HOME/.rubies/ruby-head
+	make -j16 install
+
+### Without Documentation
+
+If you are frequently building Ruby, this will reduce the time it takes to `make install`.
+
+	../configure --disable-install-doc
+
+## Running Ruby
+
+### Run Local Test Script
+
+You can create a file in the Ruby source root called `test.rb`. You can build `miniruby` and execute this script:
+
+	make -j16 run
+
+If you want more of the standard library, you can use `runruby` instead of `run`.
+
+### Run Bootstrap Tests
+
+There are a set of tests in `bootstraptest/` which cover most basic features of the core Ruby language.
+
+	make -j16 test
+
+### Run Extensive Tests
+
+There are extensive tests in `test/` which cover a wide range of features of the Ruby core language.
+
+	make -j16 test-all
+
+You can run specific tests by specifying their path:
+
+	make -j16 test-all TESTS=../test/fiber/test_io.rb
+
+### Run RubySpec Tests
+
+RubySpec is a project to write a complete, executable specification for the Ruby programming language.
+
+	make -j16 test-all test-rubyspec