Add example of symmetric matrix

Author: BenBrock

spec/latest/index.bs

@@ -381,6 +381,84 @@ with only the upper triangle stored.  For all matrix entries with row and column
 indices `i,j` and value `v`, `i <= j`.  If `i != j`, the entry implies the
 presence of an entry at row and column index `j,i` with value `-v`.
 
+<div class=example>
+
+Example of a symmetric CSR matrix.
+
+<table>
+  <thead>
+  <tr>
+    <th> </th>
+    <th>0</th>
+    <th>1</th>
+    <th>2</th>
+    <th>3</th>
+    <th>4</th>
+  </tr>
+  </thead>
+  <tbody>
+  <tr>
+    <th>0</th>
+    <td>1</td>
+    <td>.</td>
+    <td>.</td>
+    <td>.</td>
+    <td>.</td>
+  </tr>
+  <tr>
+    <th>1</th>
+    <td>2</td>
+    <td>9</td>
+    <td>.</td>
+    <td>.</td>
+    <td>.</td>
+  </tr>
+  <tr>
+    <th>2</th>
+    <td>7</td>
+    <td>.</td>
+    <td>2</td>
+    <td>.</td>
+    <td>.</td>
+  </tr>
+  <tr>
+    <th>3</th>
+    <td>.</td>
+    <td>2</td>
+    <td>.</td>
+    <td>3</td>
+    <td>.</td>
+  </tr>
+  <tr>
+    <th>4</th>
+    <td>.</td>
+    <td>.</td>
+    <td>3</td>
+    <td>.</td>
+    <td>7</td>
+  </tr>
+  </tbody>
+</table>
+
+```json
+{
+  "format": "CSR",
+  "shape": [5, 5],
+  "structure": "symmetric_lower",
+  "data_types": {
+    "pointers_0": "uint64",
+    "indices_1": "uint64",
+    "values": "int8"
+  }
+}
+```
+
+- `pointers_0` = [0, 1, 3, 5, 7, 9]
+- `indices_1` = [0, 0, 1, 0, 2, 1, 3, 2, 4]
+- `values` = [1, 2, 9, 7, 2, 2, 3, 3, 7]
+
+</div>
+
 Binary Containers {#binary_container}
 =====================================
 Binary containers must store binary arrays in a standardized, cross-platform