Add possible reasons UAs don't track "initiator_url" for certain resources. (#1173)

* Add text on improving user experience.

* Move over resource-timing-content-encoding explainer

New changes that have never been reviewed:

1. Added text in "User research" on how this feature affects end
users. (Per TAG reviewer's request);
2. Added author's github account in "author" section;
3. Added "participate" section;
4. Issue template and entries in README.

* Improvements on the explainer.

* Add reasons to skip resources.

---------

Co-authored-by: Guohui Deng <guohuideng@microsoft.com>

Author: guohuideng2024

ResourceTimingInitiatorInfo/explainer.md

@@ -70,6 +70,7 @@ There are a number of advantages to this approach.
 ### 3. UA only partially implements the `initiator Info`.
 When the `initiator info` is missing for some resources, the partial resource dependency information is still useful. Therefore, a UA can release a partial `initiator info` implementation and make improvements later.
 
+It's expected that some UAs skip resources for the following reasons: rarity of such resources; excessive complexty in implementation; resources being fetched with deprecated HTML/CSS/JS features.
 
 ## Alternatives considered