[libgpiod][PATCH v2] README: mention the Linux 5.10 requirement for libgpiod v2

[Bartosz Golaszewski <brgl@bgdev.pl>]

From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Major version 2 of libgpiod requires linux kernel uAPI v2 to be available.
This was released in Linux 5.10 so mention it in the README.

Suggested-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
v1 -> v2:
- reword as suggested by Kent

 README | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/README b/README
index 632aab3..6063f43 100644
--- a/README
+++ b/README
@@ -8,8 +8,9 @@ libgpiod
              character device (gpiod stands for GPIO device)
 
 Since linux 4.8 the GPIO sysfs interface is deprecated. User space should use
-the character device instead. This library encapsulates the ioctl calls and
-data structures behind a straightforward API.
+the character device instead. Version 2 of libgpiod requires GPIO character
+device uAPI v2 which was first released in linux v5.10. This library
+encapsulates the ioctl calls and data structures behind a straightforward API.
 
 RATIONALE
 ---------
-- 
2.40.1