docs: add source build instructions

README.md

@@ -34,6 +34,31 @@ Known platforms without wheels:
 - PowerPC
 - Windows 32bit
 
+## Building from Source
+
+If you are installing fugashi from source, install MeCab first. The build uses
+`mecab-config` on Unix-like platforms to find the MeCab headers and libraries.
+If `mecab-config` is not on your `PATH`, the build will fail with an error
+asking whether MeCab is installed.
+
+On Linux or macOS, a typical local development setup looks like this:
+
+```sh
+python -m venv .venv
+. .venv/bin/activate
+python -m pip install --upgrade pip
+python -m pip install -e '.[unidic-lite]'
+python -c "from fugashi import Tagger; print(Tagger('-Owakati').parse('麩菓子'))"
+```
+
+The editable install builds the Cython extension in place. Installing the
+`unidic-lite` extra is not required for compilation, but it gives fugashi a
+dictionary so you can immediately verify that the built package works.
+
+If you need to build from source on Windows, install MeCab from
+[@chezou's fork](https://github.com/chezou/mecab) first, then run the same
+editable install command in your Python environment.
+
 ## Usage
 
 ```python