document the optional dependencies

Explain in the installation section that a base install needs no
compiler, and give the install command for each optional feature. The
tests section said to run the suite with `uv run pytest`, which
contradicted the installation section's statement that uv is not
required; use plain pytest and note that tests for the optional features
skip when their dependency is absent.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
(cherry picked from commit 1c4c8dfe31cb8a8eea0dc72b7ca136b44e134c9f)

README.md

@@ -19,8 +19,26 @@ this repository:
 [uv](https://docs.astral.sh/uv/) works too (`uv sync`) and is convenient
 for development, but it is not required.
 
-One dependency, `pywikidiff2`, is installed from source and compiles a
-C++ extension, so a C++ compiler must be available at install time.
+This installs everything needed for the columns most people use, and
+requires no compiler. Two features carry heavier dependencies and are
+therefore optional.
+
+`--diff` and `-p wikidiff2` need
+[pywikidiff2](https://gitea.communitydata.science/groceryheist/pywikidiff2),
+a Python binding for MediaWiki's wikidiff2 diff engine. It is not on
+PyPI and compiles a C++ extension, so it needs a C++ compiler and
+libthai:
+
+    pip install 'pywikidiff2 @ git+https://gitea.communitydata.science/groceryheist/pywikidiff2.git'
+
+`-p legacy` needs `mediawiki-utilities`, which is only useful for
+reproducing results from older research projects:
+
+    pip install '.[legacy]'
+
+wikiq tells you which of these to install if you use an option that
+needs one. The other persistence methods, including the default
+`-p sequence`, need neither.
 
 Wikimedia dumps are usually compressed as 7z (most common), gz, or bz2.
 wikiq reads these by running your system's decompression tools, so it
@@ -80,12 +98,16 @@ The most commonly useful options (`wikiq --help` describes them all):
 
 From the repository root:
 
-    uv run pytest test/
+    pip install pytest pandas pytest-asyncio pytest-benchmark
+    pytest test/
 
 Run the suite from the root—some tests open fixture files by paths
 relative to it. The full suite processes several real dumps from
-`test/dumps/` and takes a couple of minutes; expected outputs live in
-`test/baseline_output/`.
+`test/dumps/`; expected outputs live in `test/baseline_output/`.
+
+Tests covering `--diff`, `-p wikidiff2`, and `-p legacy` skip when their
+optional dependency is absent, so a base install reports skips rather
+than failures.
 
 ## Authors