Update README with detailed installation and usage instructions (#13)

This PR significantly expands the README.md file to provide more
comprehensive information about the `sum-diff` tool. The changes
include:

1. Enhanced project description:
- Clarified that `sum-diff` analyzes both branch name and code changes

2. Improved installation instructions:
   - Added step-by-step commands for installation
   - Included option to install nightly version from GitHub

3. New API key section:
   - Explained the requirement for an Anthropic API key
- Provided instructions on how to set the API key as an environment
variable

4. Expanded usage section:
   - Added more context on when and how to use the tool
   - Included an example of the tool's output

5. Formatting improvements:
   - Added more code blocks and examples for better readability

These updates aim to make it easier for users to understand, install,
and use the `sum-diff` tool effectively. The README now provides a more
complete overview of the project and its requirements.

Author: ishikawa

README.md

@@ -2,17 +2,64 @@
 
 > Summarize your changes into a concise title and a detailed description.
 
-In this project, we provide a simple command called `sum-diff`. This command automatically generates a concise and clear title and a detailed description from the current branch's differences in a git repository.
+`sum-diff` is a simple command-line tool that automatically generates a concise title and a detailed description by analyzing the current git branch, including:
 
-## Install
+- Branch name
+- Code changes (`git diff`)
+
+## Installation
+
+Install using `pipx`:
 
 ```bash
+# Download and unzip
 $ unzip sum-diff.zip
-$ pipx ./sum-diff
+$ pipx install ./sum-diff
+
+# Install nightly from GitHub
+$ pipx install git+https://github.com/ishikawa/sum-diff
 ```
 
-## How to use
+## API Key
+
+`sum-diff` uses Anthropic's Claude 3.5 Sonnet to generate the output. Obtain an API key from [Anthropic's console](https://console.anthropic.com/) and set it as an environment variable:
+
+```bash
+$ export ANTHROPIC_API_KEY=sk-ant-...
+```
+
+## Usage
+
+After making changes to your project and before creating a PR, run the `sum-diff` command to have the AI generate a title and description for your PR.
 
 ```bash
 $ sum-diff
 ```
+
+The command will take a few seconds to process and then output the title and description. For example:
+
+````
+Add `while` statement support to compiler
+
+This PR introduces support for `while` loops in the compiler, enhancing the language's control flow capabilities.
+
+Key changes:
+- Add `While` token and parsing logic in `tokenizer.rs`
+- Implement `WhileStmt` struct and parsing in `parser.rs`
+- Add code generation for `while` loops in `asm.rs`
+- Update `LocalVarAllocator` to handle `while` statements in `allocator.rs`
+- Add test cases for `while` loops in `test.sh`
+
+The implementation follows the existing pattern for control flow statements, such as `if` statements. The `while` loop consists of a condition and a body, which are evaluated and executed accordingly.
+
+Example of the new `while` loop syntax:
+
+```rust
+i = 0;
+while (i < 20)
+  i = i + 1;
+return i;
+```
+
+This PR completes the basic control flow structures for the compiler, allowing for more complex program logic to be expressed in the language.
+````