btrfs-progs: README: reword DCO/Signed-off-by requirements

The Signed-off-by is not mandatory for changes to encourage
contributions. As we'd still want to keep some quality level of final
commits the patches may be updated. For that the Signed-off-by may be
added even if there's no other such tag in the original contribution.

[ci skip]

Signed-off-by: David Sterba <[email protected]>

Author: kdave

README.md

@@ -105,10 +105,11 @@ The development model of btrfs-progs shares a lot with the kernel model. The
   (typos, documentation) but is desired as this documents who authored the
   change, you can read more about the
   [The Developer's Certificate of Origin (chapter 11)](https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin)
-  * if you are not used to the signed-off style, your contributions won't be
-    rejected just because it's missing, the _Author:_ tag will be added as a
-    substitute in order to allow contributions without much bothering with
-    formalities
+  * if you are not used to the signed-off-by style, your contributions won't be
+    rejected just because it's missing, some other way of attribution will be
+    used, like a link to issue or pull request to allow contributions without
+    much bothering with formalities
+  * project maintainers will add their Signed-off-by tag regardless
 
 ### Pull requests