Add some documentation.

Signed-off-by: Krister Johansen <krister.johansen@oracle.com>

docs/kernel-bcc.md

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+# Using the bcc utility with LinuxKit
+
+The `bcc` utility is a standard Linux tool to access performance
+counters, trace events and access various other kernel internals for
+performance analysis.
+
+The `bcc` utility needs to matched be with the kernel. For recent
+kernel build, LinuxKit provides a `linuxkit/kernel-bcc` package with
+a matching tag for each kernel under `linuxkit/kernel`.
+
+The preferred way of using the `linuxkit/kernel-bcc` package is to
+add it to the `init` section. This adds `/usr/share/bcc` to the
+  systems' root filesystem. From there it can be
+  - bind mounted into your container
+  - accessed via `/proc/1/root/usr/share/bcc/tools` from with in the `getty`
+    or `ssh` container.
+  - accessed via a nsenter of `/bin/ash` of proc 1.
+
+If you want to use `bcc` you may also want to remove the `sysctl`
+container, or alternatively, disable the kernel pointer restriction it
+enables by default:
+
+```
+echo 0 > /proc/sys/kernel/kptr_restrict
+```
+
+Now, `bcc` is ready to use. The LinuxKit `bcc` package contains
+the `bcc` binary, example and tool scripts, and kernel headers for the
+associated kernel build.
+
+