Added spec for loading indicator.

loading-indicator/README.md

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+# Loading indicator
+
+This is the requirements doc for loading indicators to be implemented on the LIS Jekyll website.
+It's a very simple spec intended to force consistency across the site.
+
+## Specification version
+Version: 1.0.0
+
+<details>
+
+An initial implementation of the loading indicator functionality was completed in May of 2023 (see [Web Components PR #113](https://github.com/legumeinfo/web-components/pull/113)).
+
+</details>
+
+## Input
+
+- something is being loaded
+
+### Mockup
+
+None
+
+## Output
+
+- The part of the page data is being for is covered by a semi-opaque element
+- The [UIkit spinner](https://getuikit.com/docs/spinner) is placed at the vertical and horizontal center of the covering element
+- An alert element can be used to indicate loading status, e.g. load failed, load success, something else
+
+### Mockup
+
+None
+
+## Implementation notes
+
+None