rudimentary docs

README.md

@@ -8,4 +8,52 @@ This is work in progress and may still change.
 
 ## Documentation
 
-Documentation is incomplete at this time.
+Documentation is incomplete at this time.
+
+## Example
+
+### PMTilesReader
+
+Currently this package only exports a single `PMTilesReader` struct which is designed to read a range of bytes (like HTTP range headers). This code was originally written for use with the [sfomuseum/swifter-protomaps](https://github.com/sfomuseum/swifter-protomaps) package and has been made in to a standalone package so that it can be used with other HTTP server implementations.
+
+By default it operates on PMTiles database files using the `Foundation.FileHandle` class. While suitable for most situations, if you are working with very large databases (like the [120GB global database](https://maps.protomaps.com/builds/)) in an iOS context this will trigger "Cannot allocate memory" errors. To account for this it is possible to signal that the `PMTilesReader` instance should use a `System.FileDescriptor` instance for working with PMTiles database files.
+
+```
+import PMTiles
+import Logging
+
+guard let db_url = URL(string: "/path/to/db.pmtiles") else {
+	throw SomeErrorHere
+}
+
+var logger = Logger("label": "example")
+logger.logLevel = .debug
+
+var reader_opts = PMTilesReaderOptions(db_url, use_file_descriptor: true)
+reader_opts.Logger = logger
+
+var reader: PMTilesReader
+
+do {
+	reader = try PMTilesReader(reader_opts)
+} catch {
+	throw error
+}
+
+if case .failure(let error) = reader.Read(from: 0, to: 1024) {
+	throw error
+}
+   
+if case .failure(let error) = reader.Size() {
+	throw error
+}
+        
+if case .failure(let error) = reader.Close() {
+	throw error
+}
+```
+
+## See also
+
+* https://docs.protomaps.com/
+* https://maps.protomaps.com/builds/