Add docs for Hugo and ignore locally generated resources (#5)

Author: rolfkleef

.gitignore

@@ -0,0 +1 @@
+resources/_gen/

README.md

@@ -1,2 +1,18 @@
-## README
-Code repository for XSpec website.
+# README
+Repository for XSpec website [xspec.io](https://xspec.io).
+
+The default branch is the `hugo` branch, containing the sources of the actual website.
+
+To run a local version:
+
+* [Install Hugo following their instructions](https://gohugo.io/getting-started/installing/).
+* Clone or download this repository from GitHub.
+* Checkout the branch `hugo`: `git checkout hugo`
+* Install the git submodules: `git submodule update --init --recursive`
+* Run `hugo server -D` to get a local version of the website.
+
+To submit updates: create a pull request to the `hugo` branch.
+
+## Background
+
+The website is generated as a static site using [Hugo](https://gohugo.io/). The static site is stored in the `master` branch, and GitHub publishes the static site.