added custom opacks md

docs/custom-packs.md

@@ -1 +1,95 @@
-# Custom Packs
+## Custom Packs
+
+ForeignThon lets you extend or override any installed language pack locally — no PyPI account, no new package required. You can also scaffold a completely new language from scratch.
+
+---
+
+## Local override
+
+Create a `custom.json` in your project root with only the keys you want to change:
+
+```json
+{
+  "builtins": {
+    "show": "print"
+  },
+  "keywords": {
+    "when": "if"
+  }
+}
+```
+
+Then reference it in `.foreignthon.toml`:
+
+```toml
+[foreignthon]
+lang = "es"
+custom_pack = "custom.json"
+```
+
+Custom keys are merged on top of the installed pack. Installed pack keys are preserved — only the keys you define in `custom.json` are overridden.
+
+!!! tip
+    ForeignThon walks up the directory tree to find `.foreignthon.toml`, so you can place it at the project root and run `fpy` from any subdirectory.
+
+---
+
+## Scaffold a new language
+
+If no pack exists for your language yet:
+
+```bash
+fpy new myproject --lang custom
+```
+
+You will be prompted for:
+
+- Language code (e.g. `ru`, `fr`, `ar`)
+- English name (e.g. `Russian`)
+- Native name (e.g. `Русский`)
+
+This generates a `custom.json` based on the official template — every Python keyword, builtin, exception, and stdlib module is listed with the English value as a placeholder. Replace the **keys** with your language's words.
+
+```json
+{
+  "keywords": {
+    "if": "if",      ← replace the key, keep the value
+    "for": "for",
+    "def": "def",
+    ...
+  }
+}
+```
+
+The `.foreignthon.toml` is automatically wired to use this file.
+
+---
+
+## Pack schema
+
+A standalone pack must have these top-level sections:
+
+| Section | Purpose |
+|---|---|
+| `meta` | Name, code, version, authors |
+| `keywords` | Python reserved words |
+| `builtins` | Built-in functions |
+| `exceptions` | Built-in exception classes |
+| `error_messages` | Translations for bilingual error output |
+| `stdlib` | Common standard library module names |
+| `postfix_keywords` | English keywords to rewrite in `--postfix` output |
+
+Validate your pack at any time:
+
+```bash
+fpy pack custom.json
+# ✓ Pack 'Russian' is valid.
+```
+
+---
+
+## Publishing
+
+Once your `custom.json` is complete and working, you can turn it into a proper `foreignthon-xx` package on PyPI so others can install it with `pip install foreignthon-xx`.
+
+See [Contributing → Language Packs](contributing/language-packs.md) for the full guide. Custom Packs