Add README for ext/ directory explaining its existence/usage
diff --git a/ext/README.md b/ext/README.md
new file mode 100644
index 0000000..874dd1b
--- /dev/null
+++ b/ext/README.md
@@ -0,0 +1,14 @@
+# libavif ext dir
+
+This contains references to various external repositories which are known to
+build and work properly with the current libavif. If you are building for
+Windows or any kind of fully static (embedded) release, using these scripts in
+conjunction with the `BUILD_SHARED_LIBS=0` and `AVIF_LOCAL_*` CMake flags make
+for a convenient way to get all of the dependencies necessary. This method is
+how many of libavif's continuous builders work.
+
+However, if you are building this for a distribution or Unix-like environment
+in general, you can safely *ignore* this directory. Allow `BUILD_SHARED_LIBS`
+to keep its default (`ON`), enable the appropriate `AVIF_BUILD_*` CMake flags
+depending on which shared AV1 codec libraries you plan to leverage and depend
+on, and use the system's zlib and libpng.
