FAQ: Document kernel feature support.

Some recent features have more stringent requirements for kernel
versions than the FAQ describes. Add an entry to be more explicit on
which features work with which versions of the upstream kernel.

Signed-off-by: Joe Stringer <[email protected]>
Acked-by: Pravin B Shelar <[email protected]>

FAQ.md

@@ -165,6 +165,29 @@ A: The following table lists the Linux kernel versions against which the
    It should build against almost any kernel, certainly against 2.6.32
    and later.
 
+### Q: Are all features available with all versions of the kernel module?
+
+A: Features are gradually introduced into the upstream kernel so unless
+   you are running the latest Linux release there may be some missing
+   features. ovs-vswitchd probes for kernel features during startup and
+   lists them in the ovs-vswitchd log file. If you need features that
+   are unsupported, you can upgrade the Linux version or use the kernel
+   module paired with the userspace distribution.
+
+   For a given Open vSwitch release, the vast majority of features are
+   supported when using the kernel module paired with the userspace
+   distribution, no matter which kernel you build it against. However,
+   from time to time, new features may be introduced which rely on
+   functionality not available in older kernels. The features below are
+   included in the kernel module distributed with OVS, but have more
+   stringent requirements for minimum kernel versions than the support
+   listed earlier.
+
+| Feature   | Open vSwitch | Linux kernel
+|:---------:|:------------:|:------------:
+|    STT    |     2.4+     |     3.5+
+| Conntrack |     2.5+     |     3.10+
+
 ### Q: I get an error like this when I configure Open vSwitch:
 
        configure: error: Linux kernel in <dir> is version <x>, but