docs(CLI): add installation instructions (#1417)

Signed-off-by: Juan Cruz Viotti <jviotti@openmailbox.org>

docs/CLI.md

@@ -8,28 +8,64 @@ even used from a script.
 In fact, the Etcher desktop application is simply a wrapper around the CLI,
 which is the place where the actual writing logic takes place.
 
+Installing
+----------
+
+Head over to [etcher.io][etcher] and download the package that corresponds to
+your operating system.
+
+macOS and GNU/Linux
+-------------------
+
+- Extract the `.tar.gz` package by running:
+
+```sh
+tar fvx path/to/cli.tar.gz
+```
+
+- Move the resulting directory to `/opt/etcher-cli`
+
+- Add `/opt/etcher-cli` to the `PATH`. For example, add the following to
+  `.bashrc` or `.zshrc`:
+
+```sh
+export PATH="$PATH:/opt/etcher-cli"
+```
+
+Windows
+-------
+
+- Unzip the `.zip` package by right-clicking on it and selecting "Extract All"
+
+- Move the resulting directory to `C:\etcher-cli`
+
+- Add `C:\etcher-cli` to the `%PATH%`
+
+  - On Windows 10 and Windows 8
+    - Open *Control Panel*
+    - Open *System
+    - Click the *Advanced system settings* link
+    - Click *Environment Variables*
+    - Find the `PATH` environment variable, and click *Edit*
+    - Append `;C:\etcher-cli` to the environment variable value
+    - Click *OK*
+
+  - On Windows 7
+    - Right-click the *My Computer* icon
+    - Open the *Properties* menu
+    - Open the *Advanced* tab
+    - Click *Environment Variables*
+    - Find the `PATH` environment variable, and click *Edit*
+    - Append `;C:\etcher-cli` to the environment variable value
+    - Click *OK*
+
+  - Re-open `cmd.exe`, or PowerShell
+
 Running
 -------
 
-We are not oficially releasing the Etcher CLI as a separate package yet, but
-you can run it locally with the following steps:
-
-- Clone the Etcher repository.
-
-```
-git clone https://github.com/resin-io/etcher
-```
-
-- Install the dependencies by running:
-
 ```sh
-npm install
-```
-
-- Run the Etcher CLI from `bin/etcher`.
-
-```
-./bin/etcher --help
+etcher -v
 ```
 
 Options
@@ -49,3 +85,5 @@ Debug mode
 
 You can set the `ETCHER_CLI_DEBUG` environment variable to make the Etcher CLI
 print error stack traces.
+
+[etcher]: https://etcher.io/