add header image + improve css build README

.gitignore

@@ -0,0 +1 @@
+/static/css/.sass-cache

static/css/README.md

@@ -1,2 +1,10 @@
 # How to edit styles
 Do not edit css files directly. Always make your changes in the scss files and then compile them to css.
+
+## Requirement
+- dart-sass
+
+## Compiling
+```sh
+sass style.scss:style.css
+```

static/css/style.css

@@ -203,6 +203,9 @@ a:focus {
 
 header {
   position: relative;
+  background-image: url("../img/header_img_placeholder.jpg");
+  background-repeat: no-repeat;
+  background-size: cover 100%;
   padding: 1.5em 1em;
 }
 

static/css/style.css.map

@@ -0,0 +1 @@
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static/css/style.scss

@@ -44,6 +44,9 @@ a:focus {
 
 header {
   position: relative;
+  background-image: url("../img/header_img_placeholder.jpg");
+  background-repeat: no-repeat;
+  background-size: cover 100%;
   padding: 1.5em 1em;
 }