Add text to README and .gitignore

.gitignore

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+# IDEs and editors
+/.idea
+/.vscode
+
+# General
+/tmp
+/_tmp
+.cache
+
+# Python
+__pycache__/
+*.py[cod]

README.md

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 # shuriken
 
-A simplistic C++ build tool based on Ninja with first-class support for C++20 modules.
+A simplistic C++ build tool based on [Ninja](https://ninja-build.org/) with first-class support for C++20 modules.
+
+It's written in Python with minimal dependencies (so far only PyYAML) and as a single file, to allow for easy integration into a project and quick customizability if needed.
+
+
+## Design goals
+
+First and foremost, the goal of this project is to have a simple C++ build script for my own personal projects. It is **not** designed as a general purpose build system, but specifically tailored to my own use cases.
+
+That being said, feel free to use it in your own projects if it works for you, and tell me your feedback.
+
+The main design goals are as follows:
+
+- Keep it simple by relying on conventions rather than supporting every possible use case.
+- Support C++20 modules for structuring code without header files, but without supporting every possible use case of modules.
+- Allow building for multiple targets (e.g. Linux, Windows, WebAssembly).
+- Allow overriding config values with a local override file instead of modifying the build config.
+- Autodetect source files instead of listing them all manually.
+
+
+## Requirements / conventions
+
+Projects need to follow some rules to allow building them with shuriken.
+
+- All source code must be in a source directory (defaults to `src`, configurable with `source_dir`).
+- All build artifacts will be created in a build directory (defaults to `build`, configurable with `build_dir`) with subdirectories for each target (e.g. `build/linux/`).
+- C++ source files must use the extension `.cppm` if they define a module, `.cpp` otherwise.
+- Modules must be defined in source files using the same path and name as the module, replacing `.` with `/`. (For example, the module `foo.bar.baz` must be defined in `src/foo/bar/baz.cppm`.)
+- Modules can only be defined in one file. Splitting module interface and implementation is not supported.
+- Clang should be used as the compiler for the default target. While the compiler itself is configurable, the build dependency detection for modules uses `clang-scan-deps`. Other targets can use other compilers, since dependency detection is always done using the default target.