Add R6RS to the usage section

README.md

@@ -23,6 +23,12 @@ STklos.
 
 ## Usage
 
+### R6RS
+
+Replace the .scm file with .sps file in the next section.
+
+### R7RS
+
 You need to install each Scheme implementation yourself.
 
 The environment variable COMPILE\_R7RS must be set to the **name** of the
@@ -41,7 +47,7 @@ To get the list of all supported implementations run:
 
     compile-r7rs --list-schemes
 
-Then run it with the .scm file for r7rs, or .sps file for r6rs.
+Then run it with the .scm file for R7RS, or .sps file for R6RS.
 
     COMPILE_R7RS=<implementation name> compile-r7rs -I . -o main main.scm