Modification of CONTRIBUTING.md

Some minor fixes of the  CONTRIBUTING.md file.

CONTRIBUTING.md

 # How to contribute
 
-mruby is an open-source project which is looking forward to each contribution. 
+mruby is an open-source project which is looking forward to each contribution.
 
 ## Your Pull Request
 
@@ -8,10 +8,10 @@ To make it easy to review and understand your change please keep the following
 things in mind before submitting your pull request:
 
 * Work on the latest possible state of **mruby/master**
-* Test your changes before creating a pull request (**make test**)
+* Create a branch which is dedicated to your change
+* Test your changes before creating a pull request (```./minirake 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 your change
 * Create a meaningful commit message
 * Explain your change (i.e. with a link to the issue you are fixing)
 
@@ -27,16 +27,16 @@ C code:
 
 #### Comply with C99 (ISO/IEC 9899:1999)
 
-mruby should be highly portable to other systems and compilers. For that it is
+mruby should be highly portable to other systems and compilers. For this it is
 recommended to keep your code as close as possible to the C99 standard
 (http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf).
 
-Although we target C99, VC is also an important target for mruby, so that we
-avoid local variable declaration in the middle.
+Although we target C99, Visual C++ is also an important target for mruby, so
+you should local variable declarations in the middle.
 
 #### Reduce library dependencies to a minimum
 
-The dependencies to libraries should be put to an absolute minimum. This
+The dependencies to libraries should be kept to an absolute minimum. This
 increases the portability but makes it also easier to cut away parts of mruby
 on-demand.
 
@@ -56,7 +56,7 @@ Use C++ style comments only for temporary comment e.g. commenting out some code
 
 ### Ruby code
 
-Parts of the standard library of mruby is written in the Ruby programming language
+Parts of the standard library of mruby are 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)