Add a contribution information file

CONTRIBUTING.md

@@ -1,14 +1,33 @@
-# Coding conventions
+# How to contribute
+
+mruby is a young open-source project.  
+
+We are looking forward to each pull request. 
+
+## Pull Request
+
+To make it easy to review und understand your change please keep the following
+things in mind for your pull request:
+
+* Make you change on the latest possible state of +mruby/master+
+* Test your changes before creating a pull request (+make test+)
+* If possible write a test case which confirms your change
+* Don't mix several features or bug-fixes in one pull request
+* Create a branch which is dedicated to change
+* Create a meaningful commit message
+* Explain your change (i.e. with a link to the issue you are fixing)
+
+## Coding conventions
 
 How to style your C and Ruby code which you want to submit.
 
-## C code
+### C code
 
 The core part (parser, bytecode-interpreter, core-lib, etc.) of mruby is
 written in the C programming language. Please note the following hints for your
 C code:
 
-### Comply with C99 (ISO/IEC 9899:1999)
+#### Comply with C99 (ISO/IEC 9899:1999)
 
 mruby should be highly portable to other systems and compilers. For that it is
 recommended to keep your code as close as possible to the C99 standard
@@ -17,19 +36,19 @@ recommended to keep your code as close as possible to the C99 standard
 Although we target C99, VC is also an important target for mruby, so that we
 avoid local variable declaration in the middle.
 
-### Reduce library dependencies to a minimum
+#### Reduce library dependencies to a minimum
 
 The dependencies to libraries should be put to an absolute minimum. This
 increases the portability but makes it also easier to cut away parts of mruby
 on-demand.
 
-### Don't use C++ style comments
+#### Don't use C++ style comments
 
     /* This is the prefered comment style */
 
 Use C++ style comments only for temporary comment e.g. commenting out some code lines.
 
-### Insert a break after the method return value:
+#### Insert a break after the method return value:
 
     int
     main(void)
@@ -37,12 +56,12 @@ Use C++ style comments only for temporary comment e.g. commenting out some code
       ...
     }
 
-## Ruby code
+### Ruby code
 
 Parts of the standard library of mruby is written in the Ruby programming language
 itself. Please note the following hints for your Ruby code:
 
-### Comply with the Ruby standard (ISO/IEC 30170:2012)
+#### Comply with the Ruby standard (ISO/IEC 30170:2012)
 
 mruby is currently targeting to execute Ruby code which complies to ISO/IEC
 30170:2012 (http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=59579).