update preface

@tvandersar I've edited the front page, check it out and let me know what you think.

src/index.md

-# Delft Solid State Physics course
+# Open Solid State Notes
 
-If something is unclear, if you notice a typo, or if you have a suggestion, please [let us know](https://gitlab.kwant-project.org/solidstate/lectures/issues/new?issuable_template=typo)!
+!!! summary "Learning goals"
 
-## Course book
+    After following this course you will be able to:
 
-The Oxford Solid State Basics by Steven H. Simon
+    - Formulate the main models of solid state physics
+    - Given a description of a solid state physics problem, write down the corresponding equations
+    - Identify the relevant approximations and apply them to solve this problem
+
+In these notes our aim is to provide learning materials which are:
+
+- self-contained
+- easy to modify and remix, so we provide the full source, including the code
+- open for reuse: see the license below.
+
+Whether you are a student taking this course, or an instructor reusing the materials, we welcome all contributions, so check out the [course repository](https://gitlab.kwant-project.org/solidstate/lectures), especially do [let us know](https://gitlab.kwant-project.org/solidstate/lectures/issues/new?issuable_template=typo) if you see a typo!