Add PR references

Author: k0kubun

NEWS.md

@@ -2,11 +2,11 @@
 
 ## 4.0.0
 
-* Optimize `ERB::Util.html_escape`
+* Optimize `ERB::Util.html_escape` [#27](https://github.com/ruby/erb/pull/27)
   * No longer duplicate an argument string when nothing is escaped.
      * This makes `ERB::Util.html_escape` faster than `CGI.escapeHTML` in no-escape cases.
   * It skips calling `#to_s` when an argument is already a String.
-* Define `ERB::Escape.html_escape` as an alias to `ERB::Util.html_escape`
+* Define `ERB::Escape.html_escape` as an alias to `ERB::Util.html_escape` [#38](https://github.com/ruby/erb/pull/38)
   * `ERB::Util.html_escape` is known to be monkey-patched by Rails.
     `ERB::Escape.html_escape` is useful when you want a non-monkey-patched version.
 * Drop deprecated `-S` option from `erb` command