Add README file

Makefile

@@ -1,3 +1,4 @@
+# This is based on Textern’s Makefile (https://github.com/jlebon/textern)
 ifneq ($(USER),1)
 	PREFIX ?= /usr/local
 	MOZILLA_PREFIX ?= /usr
@@ -38,4 +39,4 @@ native-uninstall:
 
 .PHONY: xpi
 xpi:
-	@rm -f shellfox.xpi && zip -r shellfox.xpi background.js _locales/ manifest.json options.* res/*.png
+	@rm -f shellfox.xpi && zip -r shellfox.xpi background.js _locales/ manifest.json options.* res/shell*.png

README.md

@@ -0,0 +1,28 @@
+# ![](res/shellfox-38.png) Shellfox
+
+[Shellfox](https://hak.xwx.moe/jadedctrl/shellfox) is a simple add-on for [Firefox](https://firefox.com) that allows you to run shell commands at the click of a button.
+
+<a alt="Screenshot of Shellfox." href="res/screenshot-youtube.png"><img width=555px src="res/screenshot-youtube.png"></a>
+
+
+## Installation
+Shellfox uses a native program in order to run commands; installing only the add-on isn’t enough. Currently, the native program has only been tested on GNU/Linux, though it is likely to work on BSDs, and potentially macOS.
+
+First, download the repository: [shellfox.zip](https://hak.xwx.moe/jadedctrl/shellfox/archive/master.zip)
+
+Then, in the repository’s directory, open a terminal and run:
+
+```
+$ sudo make native-install
+```
+
+After that, you can install the add-on; then, voilà!
+
+
+## Related projects
+This add-on was inspired by [Textern](https://github.com/jlebon/textern/), which allows you to edit text-boxes with an external editor like [Emacs](https://gnu.org/software/emacs). It’s very useful, I highly recommend it!
+
+## Source code
+https://hak.xwx.moe/jadedctrl/shellfox  
+Author is Jaidyn Ann, <jadedctrl@posteo.at>  
+License is the [GNU GPLv3](LICENSE).