restructure readme, add additional guides


been relearning some postgis and postgreSQL in general lately and found that most online resources on postgreSQL assume the reader is already familiar with SQL and most postGIS resources assume the reader is familiar with postgreSQL. =( 

I added links that I found to be more helpful for those new to SQL and postgreSQL; and I removed some slides/notes that I originally wrote and is replaced by postgis-101.md