Add to docs how to configure app for auto backup

[levibostian]

I was reading the docs for auto backup recently and it talks about how docs in getExternalStorage() are automatically backed up.

Add to docs how to exclude photos taken by Shutter.

[levibostian]

Looking over the docs for auto backup, I believe I can do something even better.

First off, as the library stands today, this is what we need to add to the README on how to exclude Shutter videos and photos from auto backup:

• In manifest, add this:

<application ...
    android:fullBackupContent="@xml/my_backup_rules">
</application>

• Create an XML file called my_backup_rules.xml in the res/xml/ directory. Inside, do:

<?xml version="1.0" encoding="utf-8"?>
<full-backup-content>
    <exclude domain="file" path="Pictures/"/>
    <exclude domain="file" path="Movies/"/>
</full-backup-content>

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Now, about how I can make this even better.

Allow users to configure Shutter to using a specific directory name for photos and videos if they choose. This will give the power of the user to put Shutter photos and videos in one directory and exclude that one to make sure that their app or other libraries do not clash together by putting files together into the same directory.