Add README

Covers quick start, the two web UIs, non-Docker build instructions, repo
layout, and the two Windows-porting gotchas (clock() semantics, DB string
widths) most likely to bite future changes.

.env.example

@@ -35,7 +35,7 @@ SHARED_LOGIN_PASS=789456
 SHARED_CHAR_PASS=456123
 SHARED_WORLD_PASS=321654
 
-# --- Web admin panel (http://<host>:8081) -----------------------------------
+# --- Web admin panel (http://<host>/admin/) ----------------------------------
 # Logs in with an existing game account; only accounts with
 # accesslevel >= ADMIN_MIN_ACCESSLEVEL are allowed in.
 # Generate your own with: python3 -c "import secrets; print(secrets.token_hex(32))"