Add CONTRIBUTING.md for posts and pages

CONTRIBUTING.md

@@ -0,0 +1,42 @@
+# Contributing
+We welcome contributions from people working on research software around Culham.
+
+This guide summarises the [Github guide](https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/adding-content-to-your-github-pages-site-using-jekyll) for adding new content to a Jekyll site. The "publishing source" is the `master` branch; anything added to `master` will be published automatically. Firstly, please create an issue and branch for the content you wish to add.
+
+There are two main types of content, pages and posts, both written in Markdown. A page is permanent, without an associated date, whereas a post has a date associated with it.
+
+## Adding a blog post
+In the `_posts` directory, create a new Markdown file of the form `YYYY-MM-DD-NAME-OF-POST.md` (it's easiest to copy and edit the `_posts/2021-03-15-welcome-to-jekyll.md` file). Then add the YAML front matter:
+```yaml
+---
+layout: post
+title: "POST TITLE"
+date: YYYY-MM-DD hh:mm:ss -0000
+categories: CATEGORY-1 CATEGORY-2
+---
+```
+
+Below this, add the Markdown content for your post.
+
+## Adding a page
+In the project root directory, create a new file in the form `PAGE-NAME.md` (or copy and edit the `about.md` file). Then add the YAML front matter:
+```yaml
+---
+layout: page
+title: "PAGE TITLE"
+permalink: /URL-PATH/
+---
+```
+
+Below this, add the Markdown content for your post.
+
+## Previewing changes
+Once you've set up Jekyll locally (see the `README.md`), you can preview your changes using:
+```
+bundle exec jekyll serve
+```
+
+and navigating to http://localhost:4000.
+
+## Publishing
+Open a pull request for the changes on your branch. Once merged to `master`, the content will be published on [researchsoftwareculham.github.io](https://researchsoftwareculham.github.io/), which is public.

README.md

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 # ResearchSoftwareCulham.github.io
 A blog for Research Software at Culham.
+
+## Contributing a blog post or page
+Please see the `CONTRIBUTING.md` for guidance on adding content.