Improve Gem Readme file

doc/mrbgems/README.md

@@ -5,6 +5,8 @@ standardized way into mruby.
 
 ## GEM Structure
 
+The maximal Gem structure looks like this:
+
 +- GEM_NAME         <- Name of Gem
    |
    +- mrblib/       <- Source for Ruby extension
@@ -17,8 +19,40 @@ standardized way into mruby.
    |
    +- README.md     <- Readme for Gem
 
+The folder *mrblib* contains pure Ruby files to extend mruby. The folder *src*
+contains C files to extend mruby. The folder *test* contains pure Ruby files
+for testing purposes which will be used by mrbtest. The *Makefile* contains
+rules to build all C files and integrates them into the normal mruby
+build process. +README.md+ is a short description for the Gem.
+
 ## C Extension
 
+mruby can be extended with C. It is possible by using the C API to integrate C
+libraries into mruby. You need to use the folder *src* for all C files. Pay
+attention that your *Makefile* has to build the source and also add the object
+files to libmruby.a
+
+### Pre-Conditions
+
+mrbgems will automatically call the +gem-all+ make target of your Gem. Make
+sure that you build all files in this target and that you add you object
+files to libmruby.a
+
+mrbgems expects that you have implemented a C method called
+*mrb_YOURGEMNAME_gem_init(mrb_state* mrb)*. YOURGEMNAME will be replaced
+by the name of you Gem. The directory name of your Gem is considered also
+as the name! If you call your Gem directory *c_extension_example*, your
+initialisation method could look like this:
+
+  void
+  mrb_c_extension_example_gem_init(mrb_state* mrb) {
+    _class_cextension = mrb_define_module(mrb, "CExtension");
+    mrb_define_class_method(mrb, _class_cextension, "c_method", mrb_c_method, ARGS_NONE());
+  }
+
+mrbgems will also use the *gem-clean* make target to clean up your Gem. Implement
+this target with the necessary rules!
+
 ### Example
 
 +- c_extension_example/
@@ -37,6 +71,16 @@ standardized way into mruby.
 
 ## Ruby Extension
 
+mruby can be extended with pure Ruby. It is possible to override existing
+classes or add new ones in this way. Put all Ruby files into the *mrblib*
+folder. At the moment only one directory layer is supported. So don't
+use a deeper structure for now!
+
+The *Makefile* is not used for building a Ruby extension. But you still
+should maintain this file so that during the build process the progress
+can be visualized. If you want to do additional things during the build
+process of your Ruby extension you can use the *Makefile* too.
+
 ### Example
 
 +- ruby_extension_example/