Merge branch 'preface' into 'master'

update preface

See merge request !6

mkdocs.yml

-site_name: Solid state lecture notes
+site_name: Open Solid State Notes
 repo_url: https://gitlab.kwant-project.org/solidstate/lectures
 repo_name: source
 edit_uri: edit/master/src/
@@ -38,4 +38,4 @@ extra_javascript:
   - 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.3/MathJax.js?config=TeX-AMS_HTML'
   - 'scripts/mathjaxhelper.js'
   - 'scripts/x3dom.js'
-copyright: "Copyright © 2017-2018 Delft University of Technology, CC-BY-SA 4.0."
+copyright: "Copyright © 2017-2019 Delft University of Technology, CC-BY-SA 4.0."

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!