docs: Add READMEs to subdirectories

Add short explanations to each subdirectory explaining its purpose and 
contents.

components/README.md

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+# components
+
+The `components` directory contains small re-useable PHP templates that are
+used in various place across the site.
+
+See [“Components”][docs] in the architecture documentation for more details on
+how to write and use a component.
+
+[docs]: ../ARCHITECTURE.md#components

font/README.md

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+# font
+
+The `font` directory contains the web font files bundled with the theme.
+
+The `hgnm` font provides a small number of icon glyphs used in the theme:
+
+  - `download`
+  - `facebook`
+  - `left-arrow-bold`
+  - `link-ext`
+  - `location`
+  - `right-arrow-bold`
+
+Use these with [the `icon` component][icon].
+
+If you follow [the set-up directions][su], additional fonts will be downloaded.
+
+[icon]: ../components/icon.php
+[su]: ../README.md#development

functions/README.md

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+# functions
+
+The `functions` directory contains functions that modify the WordPress install
+in various ways. These functions need to be included in [`functions.php`][fnphp]
+for WordPress to see them.
+
+[fnphp]: ../functions.php

img/README.md

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+# img
+
+The `img` directory contains image assets used by the theme.
+
+- `404.gif` is used on [the 404 error page][404]
+
+- `absurdidad.png` and `halftone.png` are used as background images.
+
+- Fallbacks are used for missing images, e.g. in [`member_list_item.php`][mli]
+
+- Favicons are used by browsers on tabs and elsewhere
+
+- `login-logo.png` is used to customise [the admin login page][login]
+
+[mli]: ../components/member_list_item.php
+[404]: ../404.php
+[login]: ../functions/custom-login-page.php

js/README.md

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+# js
+
+The `js` directory contains JavaScript files that provide theme functionality.
+
+- `main.js`
+
+  - enables the collapsible mobile menu and
+
+  - provides a simple `ready` function similar to jQuery’s `.ready()`.
+
+- The `vendor` directory contains third-party JavaScript libraries:
+
+  - [baguetteBox.js][bb] is a library providing lightbox overlays;
+
+  - [Modernizr][mzr] provides JavaScript-support detection.
+
+[bb]: https://github.com/feimosi/baguetteBox.js
+[mzr]: https://modernizr.com/

scss/README.md

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+# scss
+
+The `scss` directory contains theme style files written in [SASS][sass].
+
+`style.scss` contains the main theme style, including the important initial
+comment block that passes metadata about our theme to WordPress. It is processed
+ to generate `style.css` (see [“Build CSS”][build] in the main README).
+
+Filenames beginning with an underscore are treated as “partials” by SASS and do
+not generate their own CSS files. Instead they are imported in `style.scss` with
+the `@import` directive.
+
+`_vars.scss` contains all the variables used by `style.scss`. They can be used
+for example to a change a colour throughout the theme.
+
+[build]: ../README.md#build-css
+[sass]: https://sass-lang.com/guide