Add a new document named `mruby3.md`.

That describes the changes in `mruby3`.

doc/mruby3.md

+User visible changes in `mruby3`
+===
+
+= New Syntax
+
+We have ported some new syntax from CRuby.
+
+* R-assignment (`12 => x`)
+* Numbered block parameter (`x.map{_1 * 2}`)
+* End-less `def` (`def double(x) = x*2`)
+
+= Build System
+
+You can specify `TARGET` option to `rake` via a command line
+option, or via an environment variable, e.g.
+
+`rake TARGET=host all`
+
+or
+
+`TARGET=host rake all`
+
+It's much easier to switch multiple targets than the
+previous `build_config.rb` system.
+
+== `presym` target
+
+The first compilation of `mruby` may require generation of a
+static symbol table named `build/presym`. You can generate
+the table by `rake gensym`.
+
+== `target` directory
+
+Build target specification files are loaded from `target`
+directory. The default `TARGET` is `host` which is described
+in `target/host.rb`. There are many other targets for example:
+
+* `host-gprof`: compiles with `gprof` for performance tuning
+* `host-m32`: compiles in gcc 32bit mode on 64bit platforms
+* `boxing`: compiles all three boxing options
+* `clang-asan`: compiles with `clang`'s Address Sanitizer
+
+`target/host.rb` comes  with comments to help  writing a new
+target description.
+
+= Build Target Contribution
+
+When you write a new target description, please
+contribute. We welcome your contribution as a GitHub
+pull-request.
+
+= Configuration Options
+
+Some configuration macro names are changed for consistency
+
+== `MRB_USE_METHOD_T_STRUCT`
+
+Changed from `MRB_METHOD_T_STRUCT`.
+
+To use `struct` version of `mrb_method_t`. More portable but consumes more memory.
+Turned on by default on 32bit platforms.