Documentation: Added documentation for `cnitool`.

Added documentation to simplify the usage of `cnitool` and the
settings expected to be used with `cnitool`.

diff --git a/Documentation/cnitool.md b/Documentation/cnitool.md
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+# Overview
+The `cnitool` is a utility that can be used to test a CNI plugin
+without the need for a container runtime. The `cnitool` takes a
+`network name` and a `network namespace` and a command to `ADD` or
+`DEL`,.i.e, attach or detach containers from a network. The `cnitool`
+relies on the following environment variables to operate properly:
+* `NETCONFPATH`: This environment variable needs to be set to a
+  directory. It defaults to `/etc/cni/net.d`. The `cnitool` searches
+  for CNI configuration files in this directory with the extension
+  `*.conf` or `*.json`. It loads all the CNI configuration files in
+  this directory and if it finds a CNI configuration with the `network
+  name` given to the cnitool it returns the corresponding CNI
+  configuration, else it returns `nil`.
+
+* `CNI_PATH`: For a given CNI configuration `cnitool` will search for
+  the corresponding CNI plugin in this path.