[docs] Add a users manual for core functionalities

Author: forfudan

README.md

@@ -2,6 +2,14 @@
 
 A command-line argument parser library for [Mojo](https://www.modular.com/mojo).
 
+> **A**rguments  
+> **R**esolved and  
+> **G**rouped into  
+> **M**eaningful  
+> **O**ptions and  
+> **J**oined  
+> **O**bjects
+
 ## Overview
 
 ArgMojo provides a builder-pattern API for defining and parsing command-line arguments in Mojo. It supports:
@@ -87,6 +95,8 @@ fn main() raises:
 
 ## Usage Examples
 
+For detailed explanations and more examples of every feature, see the **[User Manual](docs/user_manual.md)**.
+
 Build the demo binary first, then try the examples below:
 
 ```bash
@@ -197,6 +207,7 @@ pixi run clean
 ```txt
 argmojo/
 ├── docs/                           # Documentation
+│   ├── user_manual.md              # User manual with detailed examples
 │   └── argmojo_overall_planning.md
 ├── src/
 │   └── argmojo/                    # Main package

docs/argmojo_overall_planning.md

@@ -206,21 +206,21 @@ pattern             # By order of add_arg() calls
 - [x] **`count` action** — `-vvv` → `get_count("verbose") == 3` (argparse `-v` counting)
 - [x] **Clean exit for --help/--version** — use `sys.exit(0)` instead of `raise Error`
 
-### Phase 3: Relationships & Validation (Next)
+### Phase 3: Relationships & Validation (for v0.1)
 
 - [x] **Mutually exclusive flags** — `cmd.mutually_exclusive(["json", "yaml", "toml"])`
 - [ ] **Flags required together** — `cmd.required_together(["username", "password"])`
 - [ ] **`--no-X` negation** — `--color` / `--no-color` paired flags (argparse BooleanOptionalAction)
 - [ ] **Aliases** for long names — `.aliases(["colour"])` for `--color`
 - [ ] **Deprecated arguments** — `.deprecated("Use --format instead")` prints warning (argparse 3.13)
 
-### Phase 4: Subcommands
+### Phase 4: Subcommands (maybe for v0.2)
 
 - [ ] **Subcommand support** — `app <subcommand> [args]` (cobra, argparse, clap)
 - [ ] **Subcommand help** — `app help <subcommand>` or `app <subcommand> --help`
 - [ ] **Global vs local flags** — flags that persist through to subcommands (cobra persistent flags)
 
-### Phase 5: Polish (Nice to Have)
+### Phase 5: Polish (nice-to-have features, may not be implemented soon)
 
 - [ ] **Typo suggestions** — "Unknown option '--vrb', did you mean '--verbose'?" (Levenshtein distance; cobra, argparse 3.14)
 - [ ] **Colored error output** — using mist library for ANSI styled errors/help
@@ -229,7 +229,7 @@ pattern             # By order of add_arg() calls
 
 ### Explicitly Out of Scope
 
-These will **not** be implemented:
+These will **NOT** be implemented:
 
 - Derive/decorator-based API (no macros in Mojo)
 - Shell completion script generation