Added a bit of doco to help new users of spatial search.
Removed extraneous apostrophe. Please kill me.
Added definition of what indexing actually is.
Fixed code formatting block (missing ```)
Ahem try again on code formatting.
Clarified/augmented geonetwork debugging section.
Merge pull request #13 from aodn/usage_doco

Added a bit of doco to help new users of spatial search.