geojson -> shp + shp -> geojson + more documentation

@clhenrick,

I found your batch scripts right when I started playing around with `ogr2ogr`.  They were really helpful!

Not sure if you're interested in incorporating these changes, but here's a summay:
* added `shp-to-geojson.sh` for converting shapefiles to geojson (for upload to GitHub)
* added `geojson-to-shp.sh` for converting geojson back to original shapefile format (for my own purposes I've specified that format as [EPSG:102667](http://spatialreference.org/ref/esri/102667/)
* added extra documentation to README.md

Just wanted to contribute back in case you and any others using this repo might be interested.