add "API docs" to readme

README.md

@@ -37,6 +37,74 @@ zigdig google.com a
 
 ## using the library
 
+```zig
+const dns = @import("dns");
+
+pub fn main() !void {
+    var name_buffer: [128][]const u8 = undefined;
+    const name = try dns.Name.fromString("ziglang.org", &name_buffer);
+
+    var packet = dns.Packet{
+        .header = .{
+            .id = dns.helpers.randomHeaderId(),
+            .is_response = false,
+            .wanted_recursion = true,
+            .question_length = 1,
+        },
+        .questions = &[_]dns.Question{
+            .{
+                .name = name,
+                .typ = .A,
+                .class = .IN,
+            },
+        },
+        .answers = &[_]dns.Resource{},
+        .nameservers = &[_]dns.Resource{},
+        .additionals = &[_]dns.Resource{},
+    };
+
+    // use helper function to connect to a resolver in the systems'
+    // resolv.conf
+
+    const conn = try dns.helpers.connectToSystemResolver();
+    defer conn.close();
+
+    try conn.sendPacket(packet);
+
+    // you can also do this to support any Writer
+    // const written_bytes = try packet.writeTo(some_fun_writer_goes_here);
+
+    const reply = try conn.receivePacket(allocator, 4096);
+    defer reply.deinit();
+
+    // you can also do this to support any Reader
+    // const packet = try dns.Packet.readFrom(some_fun_reader, allocator);
+    // defer packet.deinit();
+
+    const reply_packet = reply.packet;
+    logger.info("reply: {}", .{reply_packet});
+
+    try std.testing.expectEqual(packet.header.id, reply_packet.header.id);
+    try std.testing.expect(reply_packet.header.is_response);
+
+    // ASSERTS that there's one A resource in the answer!!! you should verify
+    // reply_packet.header.opcode to see if there's any errors
+
+    const resource = reply_packet.answers[0];
+    var resource_data = try dns.ResourceData.fromOpaque(
+        reply_packet,
+        resource.typ,
+        resource.opaque_rdata,
+        allocator
+    );
+    defer resource_data.deinit(allocator);
+
+    // you now have an std.net.Address
+    const ziglang_address = resource_data.A;
+}
+
+```
+
 **TODO docs**
 
 it is recommended to look at zigdig's source on `src/main.zig` to understand