README: clarify "unzip" instructions

The current `release.tar` (v0.2.7)  lacks a `release` directory -- all
files live at the root of the tar archive. But the README's Unzip
instructions mention `cd`ing to `~/Downloads/release`, which implies
that there is a `release` directory inside the tar.

Rather than verify with `tar --list --file ~/Downloads/release.tar` I
made a bad assumption, ran `tar xvf ./release.tar` in my `~/Downloads`,
and then had to clean up my `~/Downloads` directory.

This update clarifies that users should create the directory and extract
the tar into that directory.

README.md

@@ -21,6 +21,8 @@ or on [eBay](https://www.ebay.com/sch/i.html?_nkw=orbic+rc400l).
 2. Unzip the `release.tar`. Open the terminal and navigate to the folder
 
     ```bash
+    mkdir ~/Downloads/release
+    tar -xvf ~/Downloads/release.tar -C ~/Downloads/release
     cd ~/Downloads/release
     ```