Minor changes to Contributing

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 ## License
 
-Irmin is distributed under the ISC license. Every OCaml source file
+Irmin is distributed under the [ISC license](https://en.wikipedia.org/wiki/ISC_license). Every OCaml source file
 should start with a header comment instantiating the following
 template (use appropriate comment syntax for other languages):
 
@@ -28,13 +28,13 @@ Note that:
 
 - The holder, on the copyright line, is the name of individual
   contributors and/or their company;
-- When adding a significant new contribution to a file (i.e. more like
-  whole new features, rather than simple fixes), check whether there
+- When adding a significant new contribution to a file (i.e., more like
+  whole new features rather than simple fixes), check whether there
   already is a copyright for your copyright holder (see above):
   - If there is one, mentioning any year, it is not required to add
     the current year (but this is allowed). In no case should you
     replace the existing year with the current one.
-  - If there is no line for your copyright holder, you should add one,
+  - If there is no line for your copyright holder, you should add one
     with the current year.
 
 For example, for a source file with multiple contributors spanning