Add README.

Lest we forget what the hell this was for six months down the line.

README.md

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+`setuptools-scm-stub`
+=====================
+
+Stub for [`setuptools-scm`] that uses a hardcoded version—the value of
+`$SETUPTOOLS_SCM_PRETEND_VERSION`—instead.
+
+[`setuptools-scm`]: https://pypi.org/project/setuptools-scm
+
+Why?
+----
+
+`setuptools-scm`'s raison d'être is to extract the version from a `.git`/`.hg`
+subdirectory, which is incompatible with reproducible builds as those
+directories are not reproducible in the slightest, and is hence usually removed.
+It does, however, let you override the version with a hardcoded value too,
+albeit it's implementation thereof is rather poor.
+
+Ergo, `setuptools-scm` sans the “scm” gives you an over-engineered package that
+mutates a singular variable: one that releases much too frequently, and of—to
+put it mildly—questionable quality; this stub's purpose is simply to reduce the
+maintenance burden on packagers who have to deal with `setuptools-scm` for zero
+reason.
+
+How?
+----
+
+Simply install this package to the virtual environment in question, set the
+`SETUPTOOLS_SCM_PRETEND_VERSION` environment variable to the version you want to
+use during the build, and it should just work. This package does, however,
+assume that you're using a single virtual environment to build a single package,
+as it does not support setting the hardcoded version for a specific named
+package.