README.md: add syntax highlighting

Author: larshp

README.md

@@ -1,19 +1,19 @@
 # Assert library  
 
 Following official [documentation](https://help.sap.com/doc/abapdocu_latest_index_htm/latest/en-US/index.htm?file=abapmethods_testing.htm) we're supposed to write unit tests using following template
-```
+```abap
 cl_abap_unit_assert=>assert_equals( act = myclass=>text
                                         exp = 'X' ).
 ```
 
 The only function this library delivers - it allows us to use shorter construction like this:
-```
+```abap
 assert( myclass=>text )->equals( 'X' ).
 ```
 
 # Usage
 To use this library you need to use `ZCL_ASSERT` class as a base class for your test class. 
-```
+```abap
 CLASS ltcl_main DEFINITION INHERITING FROM zcl_assert
     FINAL FOR TESTING
   DURATION SHORT
@@ -23,7 +23,7 @@ endclass.
 ```
 
 then later in your code you can just use something like:
-```
+```abap
     assert( 1 )->not_initial( ).
     assert( 1 )->eq( 1 ).
     assert( 1 )->ne( 2 ).