list exported and GeoInterface methods in readme (#168)

* list exported methods in readme

This isn't a substitute for docs, but lacking them its nice to at least list the functions and objects people can use.

* Update README.md

Author: rafaqz

README.md

@@ -34,6 +34,146 @@ polygon = LibGEOS.union(g1, g3)
 
 GEOS functionality is extensive, so coverage is incomplete, but the basic functionality for working with geospatial data is already available. I'm learning as I go along, so documentation is lacking, but if you're interested, you can have a look at the examples in the `examples/` folder, or the tests in `test/test_geo_interface.jl` and `test/test_geos_operations.jl`.
 
+
+Conversion to/from GeoInterface objects
+---------------------------------------
+
+[GeoInterface.jl](https://github.com/JuliaGeo/GeoInterface.jl) compatible geometries from many 
+packages including GeometryBasics.jl, Shapefile.jl, GeoJSON.jl, KML.jl, ArchGDAL.jl, GADM.jl 
+and others can be easily converted to LibGEOS geometries using:
+
+```
+GeoInterface.convert(LibGEOS, other_package_geometry)
+```
+
+To broadcast this over a vector or iterable we need to use `Ref` on `LibGEOS`:
+
+```
+GeoInterface.convert.(Ref(LibGEOS), iterable_of_geometries)
+```
+
+The same applies in reverse, such as for interop with the Makie.jl/GeometryBasics.jl ecosystem:
+ 
+```
+GeoInterface.convert(GeometryBasics, libgeos_geometry)
+```
+
+For packages like Shapefile.jl and GeoJSON.jl, converting to their objects isn't 
+possible, as theyre not particularly useful on their own. Instead, we can just write directly:
+
+```
+Shapefile.write("myfile.shp", libgeos_geometry)
+```
+
+GeoInterface methods
+-----------------------
+
+GeoInterace.jl OGC standards methods work on LibGEOS objects:
+
+- `GeoInterface.distance`
+- `GeoInterface.buffer`
+- `GeoInterface.convexhull`
+- `GeoInterface.equals`
+- `GeoInterface.disjoint`
+- `GeoInterface.intersects`
+- `GeoInterface.touches`
+- `GeoInterface.within`
+- `GeoInterface.contains`
+- `GeoInterface.overlaps`
+- `GeoInterface.crosses`
+- `GeoInterface.symdifference`
+- `GeoInterface.difference`
+- `GeoInterface.intersection`
+- `GeoInterface.union`
+
+See the GeoInterface.jl [API docs](https://juliageo.org/GeoInterface.jl/stable/reference/api/) for details.
+
+Exported LibGEOS geometries:
+--------------------
+
+- `GeometryCollection`
+- `LineString`
+- `LinearRing`
+- `MultiLineString`
+- `MultiPoint`
+- `MultiPolygon`
+- `Point`
+- `Polygon`
+- `STRtree`
+
+Exported LibGEOS functions:
+-------------------
+
+- `area`
+- `boundary`
+- `buffer`
+- `bufferWithStyle`
+- `centroid`
+- `constrainedDelaunayTriangulation`
+- `containsproperly`
+- `convexhull`
+- `coveredby`
+- `covers`
+- `crosses`
+- `delaunayTriangulation`
+- `delaunayTriangulationEdges`
+- `difference`
+- `disjoint`
+- `distance`
+- `endPoint`
+- `envelope`
+- `equals`
+- `equalsexact`
+- `exteriorRing`
+- `geomLength`
+- `getGeometries`
+- `getGeometry`
+- `getPrecision`
+- `getXMax`
+- `getXMin`
+- `getYMax`
+- `getYMin`
+- `hasZ`
+- `hausdorffdistance`
+- `interiorRing`
+- `interiorRings`
+- `interpolate`
+- `interpolateNormalized`
+- `intersection`
+- `intersects`
+- `isClosed`
+- `isEmpty`
+- `isRing`
+- `isSimple`
+- `isValid`
+- `lineMerge`
+- `minimumRotatedRectangle`
+- `nearestPoints`
+- `node`
+- `numGeometries`
+- `numPoints`
+- `overlaps`
+- `pointOnSurface`
+- `polygonize`
+- `prepareGeom`
+- `project`
+- `projectNormalized`
+- `query`
+- `readgeom`
+- `setPrecision`
+- `sharedPaths`
+- `simplify`
+- `snap`
+- `startPoint`
+- `symmetricDifference`
+- `topologyPreserveSimplify`
+- `touches`
+- `unaryUnion`
+- `union`
+- `uniquePoints`
+- `within`
+- `writegeom`
+
 Installation
 ------------
 1. At the Julia prompt, run 
@@ -48,6 +188,7 @@ Installation
   ```
 
   # Ecosystem
+  * All [GeoInterface.jl](https://github.com/JuliaGeo/GeoInterface.jl) compatible geometries will work in LibGEOS without conversion.
   * Makie support for LibGEOS is available via [LibGEOSMakie.jl](https://github.com/jw3126/LibGEOSMakie.jl)
   * LibGEOS integrates with [GeoInterface.jl](https://github.com/JuliaGeo/GeoInterface.jl) making it easy to combine LibGEOS with other geometry packages.