Improve documentation for Gem and add limitations

doc/mrbgems/README.md

@@ -25,7 +25,7 @@ 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.
+build process. *README.md* is a short description of your Gem.
 
 ## C Extension
 
@@ -36,12 +36,12 @@ 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
+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 your 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
+*mrb_YOURGEMNAME_gem_init(mrb_state)*. 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:
@@ -104,3 +104,16 @@ process of your Ruby extension you can use the *Makefile* too.
    |
    +- README.md
 ```
+
+## Current Limitations
+
+The following limitations are currently existing:
+
+* Gem _MUST NOT_ have a *src* folder in case it doesn't have a 
+  C extension
+* Gem _MUST NOT_ have a *mrblib* folder in case it doesn't have a 
+  Ruby extension
+* Only Ruby files in the root directory of *mrblib* will be integrated
+
+If you have ideas how to fix these issues without implementing to much
+complexity into the code please provide your code or idea.