docs: add explainer and add example

Signed-off-by: Athan <kgryte@gmail.com>

Author: kgryte

lib/node_modules/@stdlib/blas/ext/base/gvander/README.md

@@ -59,6 +59,49 @@ The function has the following parameters:
 -   **out**: output matrix.
 -   **ldo**: stride of the first dimension of `out` (a.k.a., leading dimension of the matrix `out`).
 
+Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.
+
+<!-- eslint-disable stdlib/capitalized-comments -->
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+
+// Initial arrays:
+var x0 = new Float64Array( [ 999.0, 1.0, 2.0, 3.0 ] );
+var out0 = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );
+
+// Create offset views:
+var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 );       // start at 2nd element
+var out1 = new Float64Array( out0.buffer, out0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
+
+gvander( 'row-major', 1, 3, 3, x, 1, out, 3 );
+// out0 => <Float64Array>[ 0.0, 1.0, 1.0, 1.0, 1.0, 2.0, 4.0, 1.0, 3.0, 9.0 ]
+```
+
+When the mode is positive, the matrix is generated such that
+
+```text
+[
+    1   x_0^1   x_0^2   ...   x_0^(N-1)
+    1   x_1^1   x_1^2   ...   x_1^(N-1)
+    ...
+]
+```
+
+with increasing powers along the rows.
+
+When the mode is negative, the matrix is generated such that
+
+```text
+[
+    x_0^(N-1)   ...   x_0^2   x_0^1   1
+    x_1^(N-1)   ...   x_1^2   x_1^1   1
+    ...
+]
+```
+
+with decreasing powers along the rows.
+
 <!-- lint disable maximum-heading-length -->
 
 #### gvander.ndarray( mode, M, N, x, strideX, offsetX, out, strideOut1, strideOut2, offsetOut )