Add a README.md containing copy & paste helpers

README.md

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+# flake-compat
+
+## Usage
+
+To use, add the following to your `flake.nix`:
+
+```nix
+inputs.flake-compat = {
+  url = github:edolstra/flake-compat;
+  flake = false;
+};
+```
+
+Afterwards, create a `default.nix` file containing the following:
+
+```nix
+(import
+  (
+    let lock = builtins.fromJSON (builtins.readFile ./flake.lock); in
+    fetchTarball {
+      url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+      sha256 = lock.nodes.flake-compat.locked.narHash;
+    }
+  )
+  { src = ./.; }
+).defaultNix
+```
+
+If you would like a `shell.nix` file, create one containing the above, replacing `defaultNix` with `shellNix`.