Improve featured image (#17)

WARNING: UPGRADE HUGO TO VERSION 0.54.0 OR LATER

* post_thumbnail -> post_featured
* Add url, alt, caption and credit params for featured
* Add previewOnly param
* Add styles for featured
* Add fallback for thumbnail.*
* Add "Featured Images" page
* Show caption & credit on single pages only
* Update README: add featured image guide
* Bump Hugo min version because `reflect.IsMap` [0.38 -> 0.54.0]

2 files changed, 72 insertions, 0 deletions

diff --git a/exampleSite/content/post/featured-images/featured.png b/exampleSite/content/post/featured-images/featured.png
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+++ b/exampleSite/content/post/featured-images/featured.png
diff --git a/exampleSite/content/post/featured-images/index.md b/exampleSite/content/post/featured-images/index.md
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+---
+title: Featured Images
+description: "In this page, we will explain what featured images are and show you some examples how to add a featured \
+image for a page."
+date: 2019-09-12
+---
+
+Featured image is the image that appears on summaries at the list pages and the top of content pages. Every content page
+in this theme can have a featured image.
+
+<!--more-->
+
+## How to add a featured image
+
+Content pages in Hugo can have their images or any other files stored under the same directory of the page itself.
+In Hugo's terms, it's called **page bundle** and allows us to keep all assets together with a page content file.
+
+This theme uses page bundles for featured images. There are two main different ways to add a featured image.
+
+### Option 1. Put `featured.*` or `thumbnail.*` image in the page bundle
+
+This theme considers `featured` or `thumbnail` image in any popular graphic format in the root of the page bundle as
+a featured image.
+
+Directory structure of this page:
+
+```
+content
+└── post
+    └── featured-images // single page folder (page bundle)
+        ├── featured.png // featured image
+        └── index.md // page content file
+```
+
+### Option 2. Put any image in the page bundle & specify `featured` param
+
+You may put any image in the page bundle and specify `featured` param in the page's front matter:
+
+```yaml
+featured: image.jpg
+```
+
+- `featured` [`String`]: relative path of the image
+
+Or you can add some additional params like `alt`, `caption`, `credit` and `previewOnly`:
+
+```yaml
+featured:
+  url: image.jpg
+  alt: A scale model of the Eiffel tower standing on a map
+  caption: Eiffel tower model
+  credit: Unknown author
+  previewOnly: false
+```
+
+- `featured` [`Map`]:
+  - `url` [`String`]: relative path of the image
+  - `alt` [`String`]: alternate text for the image
+  - `caption` [`String`]: image caption
+  - `credit` [`String`]: image credit
+  - `previewOnly` [`Boolean`]: show only preview image
+
+## FAQ
+
+**Q**: _What will happen if I specify `alt`, `caption`, `credit` params, but miss `url`? Featured image will be shown or
+not?_\
+**A**: In that case, featured image would appear only if the page bundle contains `featured.*` or `thumbnail.*` image.
+Otherwise, you'll get nothing.
+
+**Q**: _Is featured image path case sensitive or insensitive? Featured.png and featured.png are different image relative
+paths for Hugo?_\
+**A**: The image matching is case-insensitive. In that case, Featured.png and featured.png are identical for Hugo.