docs(grid-list): add overview for md-grid-list

src/lib/grid-list/OVERVIEW.md

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+# md-grid-list
+
+`md-grid-list` is an alternative list view that arranges cells into grid-based layout. 
+See Material Design spec [here](https://www.google.com/design/spec/components/grid-lists.html).
+
+<!-- example(grid-list-overview) -->
+
+## Usage
+
+### Setting the number of columns
+
+An `md-grid-list` must specify a `cols` attribute which sets the number of columns in the gird. The
+number of rows will be automatically determined based on the number of columns and the number of
+items.
+
+### Setting the row height
+
+The height of the rows in a grid list can be set via the `rowHeight` attribute. Row height for the
+list can be calculated in three ways:
+
+1. **Fixed height**: The height can be in `px`, `em`, or `rem`.  If no units are specified, `px` 
+units are assumed (e.g. `100px`, `5em`, `250`).
+
+2. **Ratio**: This ratio is column-width:row-height, and must be passed in with a colon, not a
+decimal (e.g. `4:3`).
+
+3. **Fit**:  Setting `rowHeight` to `fit` This mode automatically divides the available height by
+the number of rows.  Please note the height of the grid-list or its container must be set.  
+
+If `rowHeight` is not specified, it defaults to a `1:1` ratio of width:height. 
+
+### Setting the gutter size
+
+The gutter size can be set to any `px`, `em`, or `rem` value with the `gutterSize` property.  If no 
+units are specified, `px` units are assumed. By default the gutter size is `1px`.
+
+### Adding tiles that span multiple rows or columns
+
+It is possible to set the rowspan and colspan of each `md-grid-tile` individually, using the
+`rowspan` and `colspan` properties. If not set, they both default to `1`. The `colspan` must not
+exceed the number of `cols` in the `md-grid-list`. There is no such restriction on the `rowspan`
+however, more rows will simply be added for it the tile to fill.
+
+### Tile headers and footers
+
+A header and footer can be added to an `md-grid-tile` using the `md-grid-tile-header` and
+`md-grid-tile-footer` elements respectively.