docs: Add project structure docs

Signed-off-by: Chloe M <chloe@mensia.org>

README.md

@@ -7,7 +7,16 @@ PT is built from scratch using its own independent kernel which is inspired from
 few things from UNIX (e.g., ELF, graphical boot console, etc) and omitting the bad parts (such as
 ``fork()``, etc).
 
-# Legal
+## Top-level directory structure
+
+```
+artifacts/   - Generated on build, contains the ISO among other things
+sdk/         - OS Software Development Kit
+service/     - The OS service pack (contains the kernel, apps, bootloader, etc)
+tools/       - Host specific tooling (will only work on UNIX-like hosts)
+```
+
+## Legal
 
 This project is licensed under the BSD-3 clause
 

service/ptos/README.md

@@ -0,0 +1,4 @@
+# The kernel
+
+This directory contains kernel sources which are critical for proper system operation as each subsystem manages
+resources, scheduling, etc.