Prerender: Add terminology and references sections in README

Bug: 1132746
Change-Id: Icbbf7f57b524750cb6cc2dc73506a348891b83ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497347
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821608}

content/browser/prerender/README.md

 This directory contains the experimental Prerender2 implementation
 (https://crbug.com/1126305). This feature is now behind the
-blink::features::kPrerender2 flag.
+`blink::features::kPrerender2` flag.
+
+# Summary
+
+Prerendering is "pre"-rendering, it's about pre-loading and rendering a page
+before the user actually navigates to it. The main goal of prerendering is to
+make the next page navigation faster, or ideally nearly instant.
+
+The Prerender2 is the new implementation of prerendering.
+
+# Terminology
+
+- **Activate**: The Prerender2 runs navigation code twice: navigation for
+  prerendering a page, and navigation for displaying the prerendered page.
+  "Activate" indicates the latter navigation.
+- **Legacy Prerender**: "Legacy Prerender" is the previous implementation of
+  prerendering that does not use NoStatePrefetch. This is already deprecated
+  (https://crbug.com/755921).
+- **NoStatePrefetch**: An internal mechanism for speculative prefetching of
+  pages and their subresources that are on a critical path of page loading
+  without executing any JavaScript or creating a complex state of the web
+  platform (https://www.chromestatus.com/feature/5928321099497472). This
+  mechanism is not purely "no state" because the HTTP cache allows to create
+  cookies and other state related to validating cache entries. The current
+  prerendering uses this mechanism, that is, it does not actually render pages,
+  while the Prerender2 renders pages.
+
+# References
+
+The date is the publication date, not the last updated date.
+
+- [Prerender2](https://docs.google.com/document/d/1P2VKCLpmnNm_cRAjUeE-bqLL0bslL_zKqiNeCzNom_w/edit?usp=sharing) (Oct, 2020)