CONTRIBUTING: Clarifications on how to handle license deviations

This patch makes a few clarifications which were discussed on
IRC recently.

Specifically:
- Highlight the principle that license deviations
  should be brought to the attention of maintainers
- Add a requirement for GPLv2 compatibility
- Restructure the document to highlight use-cases for
  "New components" and "Importing code" clearer
- Add conventions and instructions for "New files"

Signed-off-by: Lars Kurth <lars.kurth@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>

CONTRIBUTING

@@ -19,6 +19,19 @@ Most notably:
  - tools/xl           : LGPL v2.1
  - xen/include/public : MIT license
 
+The COMMON COPYRIGHT NOTICES section of this document contains
+sample copyright notices for the most common licenses used within
+this repository.
+
+When creating new components, new files, or importing code please follow
+the conventions outlined below. As a general rule, whenever code using a
+license other than GPLv2 is introduced, attention must be drawn to the
+difference, such that maintainers can make an informed decision about the
+deviation. Any new code must be GPLv2 compatible.
+
+New components
+--------------
+
 When creating new components and directories that contain a
 significant amount of files that are licensed under licenses other
 than GPLv2 or the license specified in the COPYING file, please
@@ -27,15 +40,21 @@ license text and a rationale for using a different license. This helps
 ensure that the license of this new component/directory is maintained
 consistently with the original intention.
 
+New files
+---------
+
+If specific files that differ from the license in a directory are introduced,
+exceptions should be highlighted and discussed in the commit message or cover
+letter introducing the file.
+
+Importing code
+--------------
+
 When importing code from other upstream projects into this repository,
 please create a README.source file in the directory the code is imported
 to, listing the original source of the code. An example can be found at
 m4/README.source
 
-The COMMON COPYRIGHT NOTICES section of this document contains
-sample copyright notices for the most common licenses used within
-this repository.
-
 Developer's Certificate of Origin
 ---------------------------------