doc: update CHANGELOG

CHANGELOG.md

@@ -88,6 +88,7 @@ This release ends support for:
 * Improve performance of some namespace operations. [[#1916](https://github.com/sparklemotion/nokogiri/issues/1916)] (Thanks, [@ashmaroli](https://github.com/ashmaroli)!)
 * Remove unnecessary array allocations from Node serialization methods [[#1911](https://github.com/sparklemotion/nokogiri/issues/1911)] (Thanks, [@ashmaroli](https://github.com/ashmaroli)!)
 * Avoid creation of unnecessary zero-length String objects. [[#1970](https://github.com/sparklemotion/nokogiri/issues/1970)] (Thanks, [@ashmaroli](https://github.com/ashmaroli)!)
+* Always compile libxml2 and libxslt with '-O2' [[#2022](https://github.com/sparklemotion/nokogiri/issues/2022), [#2100](https://github.com/sparklemotion/nokogiri/issues/2100)] (Thanks, [@ilyazub](https://github.com/ilyazub)!)
 * [JRuby] Lots of code cleanup and performance improvements. [[#1934](https://github.com/sparklemotion/nokogiri/issues/1934)] (Thanks, [@kares](https://github.com/kares)!)
 * [JRuby] Clean up deprecated calls into JRuby. [[#2027](https://github.com/sparklemotion/nokogiri/issues/2027)] (Thanks, [@headius](https://github.com/headius)!)