Ports: Update port building instructions

ports/README.md

@@ -1 +1,12 @@
-Here are all of the ports for banan-os. To build a port and run it in banan-os; add a line containing directory name from here into a file called *installed*. When you create banan-os image, all directories listed in *installed* are built.
+## Ports
+
+Here are all of the ports for banan-os.
+
+### Building
+Run the build script in the directory of the wanted port.
+```bash
+cd ports/<port name>
+./build.sh
+```
+
+Port will be installed to the disk image and is usable when running banan-os for the next time.