Update organization files

All files are adapted versions of those found in the blog already.

.gitignore

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+.bundle
+Gemfile.lock
+_site

Gemfile

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+source 'https://rubygems.org'
+gem 'github-pages', group: :jekyll_plugins

README.md

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+The main site of `image-rs` at <www.image-rs.org>.
+
+## Contributing
+
+Any member of the organization is welcome to contribute to the site, not only
+collaborators. The main branch that is served over `www.image-rs.org` should
+only be changed through PRs with at least one review, regardless of the author.
+Major changes to the structure or look should gather some feedback over a week
+but at least three days (so that one is a work day).
+
+Most pressing issues:
+
+* Design the pages instead of relying on a rather unstructured default look.
+* Establish a system for tags and categories
+* Evaluate how to present the individual projects under the organization. Each
+  projects could have one dedicated maintainer on its project page.
+
+## Serving locally
+
+For basic usage, follow [the official Github guide][GHGuide]. If you are not
+comfortable giving `sudo` to the `bundler` commands just for this project (i.e.
+this is not your main project or you created a project related account to
+confine access, ...) like me then you can also do:
+
+1. Create some local directory used to hold the installed gems. I use
+   `GEM_HOME=~/.local/lib/gems` to mirror the root tree that would otherwise be used.
+2. > `bundle install --path $GEM_HOME`
+3. Invoke jekyll through `bundle` as well
+  > `bundle exec jekyll serve`
+
+
+[GHGuide]: https://help.github.com/en/articles/setting-up-your-github-pages-site-locally-with-jekyll