Sync translated READMEs with English version (#1261)

Add Khmer (README.km.md) to all 17 other translations' language
selectors. Fix README.km.md: add translation disclaimer, download
badge in Install section, translate feature table headings/descriptions
and browser section note from English to Khmer.

README.th.md

@@ -10,7 +10,7 @@
 </p>
 
 <p align="center">
-  <a href="README.md">English</a> | <a href="README.zh-CN.md">简体中文</a> | <a href="README.zh-TW.md">繁體中文</a> | <a href="README.ko.md">한국어</a> | <a href="README.de.md">Deutsch</a> | <a href="README.es.md">Español</a> | <a href="README.fr.md">Français</a> | <a href="README.it.md">Italiano</a> | <a href="README.da.md">Dansk</a> | <a href="README.ja.md">日本語</a> | <a href="README.pl.md">Polski</a> | <a href="README.ru.md">Русский</a> | <a href="README.bs.md">Bosanski</a> | <a href="README.ar.md">العربية</a> | <a href="README.no.md">Norsk</a> | <a href="README.pt-BR.md">Português (Brasil)</a> | ไทย | <a href="README.tr.md">Türkçe</a>
+  <a href="README.md">English</a> | <a href="README.zh-CN.md">简体中文</a> | <a href="README.zh-TW.md">繁體中文</a> | <a href="README.ko.md">한국어</a> | <a href="README.de.md">Deutsch</a> | <a href="README.es.md">Español</a> | <a href="README.fr.md">Français</a> | <a href="README.it.md">Italiano</a> | <a href="README.da.md">Dansk</a> | <a href="README.ja.md">日本語</a> | <a href="README.pl.md">Polski</a> | <a href="README.ru.md">Русский</a> | <a href="README.bs.md">Bosanski</a> | <a href="README.ar.md">العربية</a> | <a href="README.no.md">Norsk</a> | <a href="README.pt-BR.md">Português (Brasil)</a> | ไทย | <a href="README.tr.md">Türkçe</a> | <a href="README.km.md">ភាសាខ្មែរ</a>
 </p>
 
 <p align="center">