Added README; added step to install BLAS in the build

.drone.yml

@@ -6,6 +6,6 @@ steps:
   - name: build
     image: fpco/stack-build:lts-20
     commands:
-      - pwd
+      - apt-get update
-      - ls
+      - apt-get install -y libopenblas-dev
       - stack build

README.md

@@ -0,0 +1,10 @@
+Petzval
+=======
+
+[![Build Status](https://ci.thequux.com/api/badges/thequux/petzval/status.svg)](https://ci.thequux.com/thequux/petzval)
+
+Petzval is a WIP lens optimization kernel. At this stage of development, it's
+probably only interesting to its author and people interested in contributing, 
+but we aim to provide an engine that can power something like Zemax or Code-V.
+
+