link.md: add new document for linking `libmruby`; ref #5393

doc/guides/link.md

+Linking `libmruby` to your application
+===
+
+You have two ways to link `libmruby` to your application.
+
+* using executable gem.
+* using normal compilation process
+
+# Executable Gems
+
+If your application is relatively small, `mrbgem` is easier way to
+create the executable.  By tradition, the gem name start with
+`mruby-bin-`, e.g. `mruby-bin-debugger`.
+
+## `mrbgem.rake` file
+
+The executable name is specified in `mrbgem.rake` file at the top of
+your `mrbgem` directory.
+
+```ruby
+MRuby::Gem::Specification.new('mruby-bin-example') do |spec|
+  spec.license = 'MIT'
+  spec.author  = 'mruby developers'
+  spec.summary = 'Example for executable command gem'
+  spec.bins = %w(mruby-example)    # <- this is binary name
+end
+```
+
+## Source tree structure
+
+The source file for the gem executable should be in
+`<gem-name>/tools/<bin-name>`.  Currently, we support C or C++ source code
+(`.c`, `.cpp`, `.cxx`, `.cc`) for the executable. Ruby source files are not
+supported. Put the functionality in the different gem and specify dependency to
+it in `mrbgem.rake`.
+
+# Normal compilation process
+
+The `libmruby` is a normal library so that you can just link it to your
+application. Specify proper compiler options (`-I` etc.) and linker options
+(`-Lmruby` etc.) to compile and link your application. Specify those options in
+your build script (e.g. `Makefile`).
+
+## Compiler options
+
+You need to specify compiler options that are compatible to mruby configuration,
+for example:
+
+* `-I` to specify the place for mruby header files
+* `-D` to specify mruby configuration macros
+
+To retrieve compiler options used to build `mruby`, you can use `mruby-config`
+command.
+
+```
+$ mruby-config --cflags
+-std=gnu99 -g -O3 -Wall -DMRB_GC_FIXED_ARENA -I/home/matz/work/mruby/include -I/home/matz/work/mruby/build/host/include
+```
+
+## Linker options
+
+Just like compiler options, you need to specify linker options that are
+compatible to mruby configuration.
+
+To retrieve linker options, you can use `mruby-config` with following options:
+
+* `--ldflags`				options passed to linker
+* `--ldflags-before-libs`   options passed to linker before linked libraries
+* `--lbs`                   linked libraries
+
+```
+$ mruby-config --ldflags
+-L/home/matz/work/mruby/build/host/lib
+
+$ mruby-config --ldflags-before-libs
+# <nothing in this case>
+
+$ mruby-config --libs
+-lmruby -lm
+```