Ignore .jekyll-cache; Add readme for git repo

.gitignore

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 _site
 .sass-cache
+.jekyll-cache/

README.md

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+# Imaginärraum Website
+
+This is the source for the [Imaginärraum-Website](https://imaginaerraum.de/).
+We use [Jekyll](https://jekyllrb.com/) to build static HTML pages from markdown source files.
+Commits to this repository will automatically trigger a rebuild of the served site.
+
+## Local development
+
+Changes can be tested locally before pushing.
+
+### Requirements
+
+* Install ruby
+* `gem install jekyll bundler`
+* `gem install jekyll-theme-minimal`
+
+### Build locally
+
+* `jekyll build`
+
+### Test locally
+
+* `jekyll serve`
+
+This opens a webserver on http://localhost:4000 and automatically rebuilds the site whenever a source file changes.