add README.md with summary and install instructions & dependencies

README.md

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+Git LaTeXdiff
+=============
+
+`git-latexdiff` is a tool to graphically visualize differences between
+different versions of a LaTeX file. Technically, it is a wrapper
+around [git](http://git-scm.com) and
+[latexdiff](https://www.ctan.org/pkg/latexdiff).
+
+If your LaTeX source is split accross several `*.tex` files,
+`git-latexdiff` can call
+[latexpand](https://www.ctan.org/pkg/latexpand) to expand it before
+running `latexdiff` (this usually works better than using `latexdiff
+--flatten`).
+
+To get the manpage (needed to get `git-latexdiff --help` to work), you
+need the `a2x` tool from the asciidoc package.
+
+Installation
+------------
+
+### Using the Makefile ###
+
+To install `git-latexdiff` in Git's exec-path and generate and install
+a manpage, run this command from the root directory of
+`git-latexdiff`'s distribution:
+
+    make install
+
+If Git was installed as root, you will need to run this command as
+root, for example using:
+
+    sudo make install
+
+If it fails, you may try installing only the script (you can still
+read the documentation using `git latexdiff -h`):
+
+    make install-bin
+
+Once the script `git-latexdiff` and the manpage `git-latexdiff.1` are
+installed, you're ready to go (no configuration file to edit).
+
+### Manual installation ###
+
+A simple way to install `git-latexdiff` is to copy the file
+`git-latexdiff` anywhere in your `$PATH` (e.g. in `/usr/local/bin`,
+run `echo $PATH` get the list) or in Git's exec-path (run `git
+--exec-path` to get it).
+
+To get the documentation, you need to generate it:
+
+    make git-latexdiff.1
+
+and then copy `git-latexdiff.1` in your man-path (run `git --man-path`
+to get the location).