[PATCH blktests 3/4] CONTRIBUTING: clarify that GPL-2.0 is not applicable

[Shin'ichiro Kawasaki]

Majority of the blktests code has GPL-3.0+, so the effective main
license is GPL-3.0+ (e.g., check, common/rc). However, GPL-2.0 and
GPL-3.0 are not compatible with each other [1]. Because the majority of
blktests is GPL-3.0+, blktests cannot contain GPL-2.0 files. All files
shall be GPL-3.0+ or GPL-2.0+. Document this requirement for
clarification.

[1] https://www.gnu.org/licenses/gpl-faq.html#v2v3Compatibility

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki at wdc.com>
---
 CONTRIBUTING.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 4cfc868..375c4eb 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -16,3 +16,7 @@ as per the [Developer Certificate of Origin](https://developercertificate.org/).
 Please run `make check` before submitting a new test. This runs the
 [shellcheck](https://github.com/koalaman/shellcheck) static analysis tool and
 some other sanity checks.
+
+When you add new files, choose a license from GPL-3.0+ or GPL-2.0+. The main
+license of blktests is GPL-3.0+. GPL-3.0+ is recommended. GPL-2.0+ is allowed
+but GPL-2.0 cannot be applied.
-- 
2.55.0