Added README instructions

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+# Git REPL
+
+## Usage
+
+Git REPL provides a Common Lisp environment containing a set of prebuilt Git-related utilities.
+It exposes two modes of operation:
+
+* Single command execution mode → Accessed by providing an S-expression as an argument, e.g. `git-repl (write-line \"hello world\")`
+* REPL mode → Accessed by executing `git-repl` without any arguments
+
+The single command execution mode allows `git-repl` to work nicely with git aliases and other standard terminal commands.
+
+Meanwhile, the REPL mode provides an environment from which you may explore the REPL and construct powerful commands to manipulate your repository.
+By virtue of providing access to Common Lisp commands, you may define your own functions and macros, or override any existing global variables.
+You can then compile a new executable with your patched changes using:
+
+    (make :executable-name "path/to/new/executable.exe" :source-file nil)
+
+## Installation
+
+To install Git REPL, install SBCL and Tcl/Tk via your package manager.
+On Windows, open Command Line or Powershell with administrator privileges, then run:
+
+    choco install -y sbcl magicsplat-tcl-tk
+
+Next, install Quicklisp using the following commands:
+
+    curl -O https://beta.quicklisp.org/quicklisp.lisp
+    sbcl --load quicklisp.lisp --eval (quicklisp-quickstart:install)
+
+You are now ready to compile Git REPL.
+
+By default, Git REPL is configured to install to `C:\bin\git-repl.exe`.
+To proceed with installing to this location, run:
+
+    sbcl --load git-repl.cl --eval (make)
+
+Otherwise, run:
+
+    sbcl --load git-repl.cl
+    (make :executable-name "path/to/new/location/git-repl.exe")
+
+Make sure `git-repl.exe` is on your path.
+You should now be able to run `git-repl` from any directory.
+
+## Git aliases
+
+Here is a list of global Git aliases (`git config --global -e`) that you may find useful for working with `git-repl`:
+
+    [alias]
+        skip = !git-repl \\(interactive-skip\\)
+        skip-all = !git-repl \\(skip-modified\\)
+        unskip-all = !git-repl \\(no-skip-all\\)
+        skip-one = update-index --skip-worktree
+        unskip-one = update-index --no-skip-worktree
+        skip-list = !git-repl \\(skipped-files\\)
+        stash-config = !git-repl \\(stash-config\\)
+        pop-config = !git-repl \\(pop-config\\)
+        apply-config = !git-repl \\(apply-config\\)
+        make-config = !git-repl \\(config-diff \\\"$1\\\"\\) #"