Explain in README how to install from git

README.md

@@ -8,3 +8,25 @@ This project is separated into two packages:
 * `libarchive-clib`: bundled C sources to avoid depending on system libarchive
 * `zlib-clib`: bundled C sources to avoid depending on system zlib
 
+## Installing from Git
+
+To build this package using Cabal directly from Git, you must run `autoreconf -i`
+in the `libarchive-clib/` subdirectory before the usual Cabal build steps
+(cabal {configure,build,install}). The program `autoreconf` is part of
+[GNU autoconf](https://www.gnu.org/software/autoconf/). There is no need to
+run the configure script: cabal configure will do this for you.
+
+If you're using a `cabal.project` you can add the following section:
+
+```
+source-repository-package
+  type: git
+  location: https://github.com/vmchale/libarchive.git
+  tag: <sha>
+  subdir: libarchive
+          libarchive-clib
+          zlib-clib
+  post-checkout-command: sh -c "cd libarchive-clib && autoreconf -i"
+```
+
+