Add simple example of glyph-designspace

To illustrate the documentation a basic example would be useful, such as this:
Example of local `glyph-designspace`
```

com.black-foundry.glyph-designspace

axes


default
20
maximum
700
minimum
20
name
height


default
20
maximum
700
minimum
20
name
width


sources


location

height
20.0
width
20.0



layername
width=200,height=700
location

height
700



layername
width=700,height=700
location

height
700
width
700



layername
width=700,height=200
location

width
700

```

The Default glyph has 3 additional layers used as sources in the designspace above:
Layer `width=200,height=700`
```

```

Layer `width=700,height=200`
```

```

Layer `width=700,height=700`
```

```