add support for exported cookies via cookies.txt, resolves #129

docs/using-cookies.md

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+# TubeSync
+
+## Advanced usage guide - using exported cookies
+
+This is a new feature in v0.10 of TubeSync and later. It allows you to use the cookies
+file exported from your browser in "Netscape" format with TubeSync to authenticate
+to YouTube. This can bypass some throttling, age restrictions and other blocks at
+YouTube.
+
+**IMPORTANT NOTE**: Using cookies exported from your browser that is authenticated
+to YouTube identifes your Google account as using TubeSync. This may result in
+potential account impacts and is entirely at your own risk. Do not use this
+feature unless you really know what you're doing.
+
+## Requirements
+
+Have a browser that supports exporting your cookies and be logged into YouTube.
+
+## Steps
+
+### 1. Export your cookies
+
+You need to export cookies for youtube.com from your browser, you can either do
+this manually or there are plug-ins to automate this for you. This file must be
+in the "Netscape" cookie export format.
+
+Save your cookies as a `cookies.txt` file.
+
+### 2. Import into TubeSync
+
+Drop the `cookies.txt` file into your TubeSync `config` directory.
+
+If detected correctly, you will see something like this in the worker or container
+logs:
+
+```
+YYYY-MM-DD HH:MM:SS,mmm [tubesync/INFO] [youtube-dl] using cookies.txt from: /config/cookies.txt
+```
+
+If you see that line it's working correctly.
+
+If you see errors in your logs like this:
+
+```
+http.cookiejar.LoadError: '/config/cookies.txt' does not look like a Netscape format cookies file
+```
+
+Then your `cookies.txt` file was not generated or created correctly as it's not
+in the required "Netscape" format. You can fix this by exporting your `cookies.txt`
+in the correct "Netscape" format.