explain GPLv3 licensing issue

static/faq.html

@@ -869,6 +869,14 @@
             <p>GrapheneOS is focused on making meaningful improvements to privacy and security,
             and bundling assorted apps into the OS is not only usually outside of that focus but
             often counter to it.</p>
+
+            <p>In some cases, licensing is also an issue. GrapheneOS is permissively licensed and
+            is usable for building devices with an immutable root of trust. GPLv3 is deliberately
+            incompatible with these kinds of locked down devices, unlike GPLv2 code such as the
+            Linux kernel. This means GrapheneOS can't include GPLv3 code without forbidding use
+            cases we want to support. GPLv3 is no problem for our own usage, but we don't want to
+            forbid using GrapheneOS as a replacement for the Android Open Source Project in locked
+            down devices.</p>
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