Update README.md

[README] Add a brief instruction on how to use

README.md

@@ -1,4 +1,16 @@
 # deathendar - the number of weeks you have left to live
 Determine the number of weeks you have left to live and print a grid of them out on a nice A4 sheet of paper! Or, perhaps, a shirt. That sounds good, too.
 
-Hope you don't mind the thimbleful of existential crisis that comes with it.
+Hope you don't mind the thimbleful of existential crisis that comes with it.
+
+*HOW TO USE:*
+1. Clone this repo.
+2.  (optional) Install virtualenv and create a virtual environment:
+```
+virtualenv venv
+. venv/bin/activate
+```
+3. Install requirements.
+`pip install -r requirements.txt`
+4. Run `python app.py`
+5. Enter your data and expect a *calendar.svg* file in the repo root directory.