docs(snap): add snap README.md

Signed-off-by: Siggi Skulason <[email protected]>

snap/README.md

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+# EdgeX CLI Snap
+[![snap store badge](https://raw.githubusercontent.com/snapcore/snap-store-badges/master/EN/%5BEN%5D-snap-store-black-uneditable.png)](https://snapcraft.io/edgex-cli)
+
+This folder contains snap packaging for the EdgeX-CLI Snap
+
+The snap currently supports both `amd64` and `arm64` platforms.
+
+## Installation
+
+### Installing snapd
+The snap can be installed on any system that supports snaps. You can see how to install 
+snaps on your system [here](https://snapcraft.io/docs/installing-snapd/6735).
+
+However for full security confinement, the snap should be installed on an 
+Ubuntu 18.04 LTS or later (Desktop or Server), or a system running Ubuntu Core 18 or later.
+
+### Installing EdgeX CLI as a snap
+The snap is published in the snap store at https://snapcraft.io/edgex-cli.
+You can see the current revisions available for your machine's architecture by running the command:
+
+```bash
+$ snap info edgex-cli
+```
+
+The latest stable version of the snap can be installed using:
+
+```bash
+$ sudo snap install edgex-cli
+```
+
+The latest development version of the snap can be installed using:
+
+```bash
+$ sudo snap install edgex-cli --edge
+```
+
+A specific version of the snap can be installed by setting the channel, for instance for 2.1 (Jakarta):
+
+```bash
+$ sudo snap install edgex-cli --channel=2.1
+```
+
+For the older CLI version compatible with 1.x of EdgeX, use `--channel=1.0`
+
+
+
+**Note** - the snap has only been tested on Ubuntu Core, Desktop, and Server.