Add 4 new screenshots to bottom of README grid

[brendancol]

The image is grid in the README is incomplete.

• Fork Xarray-Spatial
• Clone your accounts version locally git clone git@github.com:SAN154/xarray-spatial.git xarray_spatial_my_version && cd xarray_spatial_my_version
• Add Xarray-Spatial master repo as remote: git remote add upstream git@github.com:makepath/xarray-spatial.git
• Get updates from upstream repo: git fetch upstream
• Merge upstream master to your local master git merge upstream/master
• Create a new branch called git checkout -b new-screenshots
• take a few more screenshots (same size as others)...find images in example notebooks
• add them to img dir
• add them to table in README
• git add README.md and images...
• git commit -m 'added new images to README fixes #164'
• push your new branch to YOUR repo
• go to github and create Pull Request