[blog instructions] Captions, boxes (huggingface#1800)

* [blog instructions] A couple of FAQ-like recommendations

I keep referring to things like these and have to search for them every time :)

* Better example with working links in figure caption

README.md

@@ -68,3 +68,14 @@ For instance:
 
 
 ``` \\( Q(S_t, A_t) \\) ``` ➡️ $Q(S_t, A_t)$
+
+## How to Caption Figures
+
+A couple of alternatives:
+
+- Use a Markdown table [like here](https://github.com/huggingface/blog/blob/fd611f6200391b865641c4e7a38e1ea47ec4bc6c/pref-tuning.md?plain=1#L31).
+- Use HTML, [like here](https://github.com/huggingface/blog/blob/0e6ebe5ca43d6408c20eddab1f5f3205d680f3aa/watermarking.md?plain=1#L70-L73).
+
+## How to Use Highlight Boxes
+
+See an example [in this PR](https://github.com/huggingface/blog/pull/1180). This is to be used sparingly :)