Add cmd/entrypoint README.md

Signed-off-by: Vincent Demeester <[email protected]>

cmd/entrypoint/README.md

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+# entrypoint
+
+This binary is used to override the entrypoint of a container by
+wrapping it. In `tektoncd/pipeline` this is used to make sure `Task`'s
+steps are executed in order, or for sidecars.
+
+The following flags are available :
+- `-entrypoint`: "original" command to be executed (as
+  entrypoint). This will be executed as a sub-process on `entrypoint`
+- `-post_file`: file path to write once the sub-process has
+  finished. If the sub-process failed, it will write to
+  `{{post_file}}.err` instead of `{{post_file}}`.
+- `-wait_file`: file path to watch before starting the sub-process. It
+  watches for `{{wait_file}}` and `{{wait_file}}.err` precense and
+  will either execute the sub-process (in case of `{{wait_file}}`) or
+  skip the execution, write to `{{post_file}}.err` and return an error
+  (`exitCode` >= 0)
+- `-wait_file_content`: excepts the `wait_file` to add actual
+  content. It will continue watching for `wait_file` until it has
+  content.
+
+The following example of usage for `entrypoint`, wait's for
+`/builder/downward/ready` file to exists and have some content before
+executing `/ko-app/bash -- -args mkdir -p /workspace/git-resource`,
+and will write to `/builder/tools/0` in casse of succes, or
+`/builder/tools/0.err` in case of failure.
+
+```
+entrypoint \
+	-wait_file /builder/downward/ready \
+	-post_file /builder/tools/0" \
+	-wait_file_content  \
+	-entrypoint /ko-app/bash -- -args mkdir -p /workspace/git-resource
+```