examples/exec: Describe how to emulate 'exec' with 'start'

Spun off from discussion here [1].  There seemed to be consensus that
we need something like this but that it should be its own pull request
[2,3,4].

[1]: #388 (comment)
[2]: #388 (comment)
[3]: #388 (comment)
[4]: #388 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

examples/exec.md

@@ -0,0 +1,105 @@
+# Executing additional processes inside an existing container
+
+The [start operation][start] is described for creating new containers, but you can also use it to execute additional processes inside an existing container.
+This sort of execution is similar to [`nsenter`][nsenter.1]—which joins existing namespaces and executes a new process inside them—but also allows you to easily join existing [control groups][cgroups], etc.
+
+Because [start][] creates a new container, you'll need to give it a container ID, even if your new process is joining the existing container's sandbox with no additional isolation.
+
+The [start][] configuration will look like:
+
+## Example (Linux)
+
+```json
+{
+    "ociVersion": "0.5.0",
+    "platform": {
+        "os": "linux",
+        "arch": "amd64"
+    },
+    "process": {
+        "terminal": true,
+        "user": {
+            "uid": 0,
+            "gid": 0
+        },
+        "args": [
+            "sh"
+        ],
+        "env": [
+            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
+            "TERM=xterm"
+        ],
+        "cwd": "/",
+        "capabilities": [
+            "CAP_AUDIT_WRITE",
+            "CAP_KILL"
+        ]
+    },
+    "root": {
+        "path": "rootfs"
+    },
+    "linux": {
+         "cgroupsPath": "/myRuntime/myContainer",
+         "namespaces": [
+             {
+                 "type": "user",
+                 "path": "/proc/1234/ns/user"
+             },
+             {
+                 "type": "pid",
+                 "path": "/proc/1234/ns/pid"
+             },
+             {
+                 "type": "network",
+                 "path": "/proc/1234/ns/net"
+             },
+             {
+                 "type": "ipc",
+                 "path": "/proc/1234/ns/ipc"
+             },
+             {
+                 "type": "uts",
+                 "path": "/proc/1234/ns/uts"
+             },
+             {
+                 "type": "mount",
+                 "path": "/proc/1234/ns/mnt"
+             }
+         ]
+    }
+}
+```
+
+### Process
+
+You can use whichever [**`process`**][process] settings you like.
+
+### Root
+
+The [**`root.path`**][root] value is arbitrary.
+You must set a value because the field is currently [required][root], but [unless you are creating a new mount namespace the value is ignored][ignored-root].
+
+### Control group and namespace paths
+
+The [**`linux.cgroupsPath`**][cgroups] and [**`linux.namespaces[].path`**][namespaces] values can be extracted from [proc][proc.5]:
+
+1. Get the [state JSON][state] for the target container using a [state query][state-query].
+   The process ID for the target container is the **`pid`** field (e.g. `1234`).
+2. Using a [proc][proc.5] mount for the appropriate [PID namespace][pid_namespaces.7] (possibly just `/proc`), find the `{pid}` entry for the target container (e.g. `/proc/1234`).
+3. The namespace paths are in the [`ns`][proc.5] subdirectory (e.g. `/proc/1234/ns/user`, `/proc/1234/ns/pid`, …).
+4. The cgroup paths are in the [`cgroup`][proc.5] file (e.g. `/proc/1234/cgroup`).
+   Pick whichever path makes the most sense to you; containers created via OCI runtimes will [have the same path for all controllers][cgroups].
+
+[cgroups]: ../config-linux.md#control-groups
+[namespaces]: ../config-linux.md#namespaces
+[process]: ../config.md#process-configuration
+[root]: ../config.md#root-configuration
+[start]: ../runtime.md#start
+[state]: ../runtime.md#state
+[state-query]: ../runtime.md#query-state
+
+[ignored-root]: https://github.com/opencontainers/runtime-spec/pull/388#issuecomment-212188661
+
+[nsenter.1]: http://man7.org/linux/man-pages/man1/nsenter.1.html
+[proc.5]: http://man7.org/linux/man-pages/man5/proc.5.html
+[pid_namespaces.7]: http://man7.org/linux/man-pages/man7/pid_namespaces.7.html