Document ways to reshape a DataArray

Code Sample, a copy-pastable example if possible

A "Minimal, Complete and Verifiable Example" will make it much easier for maintainers to help you:
http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports

# Your code here

def xr_reshape(A, dim, newdims, coords):
    """ Reshape DataArray A to convert its dimension dim into sub-dimensions given by
    newdims and the corresponding coords.
    Example: Ar = xr_reshape(A, 'time', ['year', 'month'], [(2017, 2018), np.arange(12)]) """


    # Create a pandas MultiIndex from these labels
    ind = pd.MultiIndex.from_product(coords, names=newdims)

    # Replace the time index in the DataArray by this new index,
    A1 = A.copy()

    A1.coords[dim] = ind

    # Convert multiindex to individual dims using DataArray.unstack().
    # This changes dimension order! The new dimensions are at the end.
    A1 = A1.unstack(dim)

    # Permute to restore dimensions
    i = A.dims.index(dim)
    dims = list(A1.dims)

    for d in newdims[::-1]:
        dims.insert(i, d)

    for d in newdims:
        _ = dims.pop(-1)


    return A1.transpose(*dims)

Problem description

[this should explain why the current behavior is a problem and why the expected output is a better solution.]

It would be great to have the above function as a DataArray's method.

Expected Output

A reshaped DataArray. In the example in the function comment it would correspond to an array like

In[1] Ar.dims
Out[1]: ('year', 'month', 'lat', 'lon')

Output of xr.show_versions()

Start with the provided xr_reshape example, especially pandas.MultiIndex.from_product, DataArray.unstack(), and DataArray.transpose(). Determine where a DataArray method would fit and verify that the example produces dimensions ('year', 'month', 'lat', 'lon') when the reshape is complete.