Extend README

Add hint about file descriptor limits and add the new translation
platform.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

README.md

@@ -8,6 +8,7 @@ HackMD Community Edition
 [![build status][travis-image]][travis-url]
 [![version][github-version-badge]][github-release-page]
 [![Help Contribute to Open Source][codetriage-image]][codetriage-url]
+[![POEditor][poeditor-image]][poeditor-url]
 
 HackMD lets you create realtime collaborative markdown notes on all platforms.
 Inspired by Hackpad, with more focus on speed and flexibility.
@@ -59,6 +60,7 @@ Thanks for using! :smile:
 - Database (PostgreSQL, MySQL, MariaDB, SQLite, MSSQL) use charset `utf8`
 - npm (and its dependencies, especially [uWebSockets](https://github.com/uWebSockets/uWebSockets#nodejs-developers), [node-gyp](https://github.com/nodejs/node-gyp#installation))
 - For **building** HackMD we recommend to use a machine with at least **2GB** RAM
+- (optional) *For development you may need to increase the number of allowed open file descriptors on your machine*
 
 ### Instructions
 
@@ -327,3 +329,5 @@ See more at [http://operational-transformation.github.io/](http://operational-tr
 [standardjs-url]: https://github.com/feross/standard
 [codetriage-image]: https://www.codetriage.com/hackmdio/hackmd/badges/users.svg
 [codetriage-url]: https://www.codetriage.com/hackmdio/hackmd
+[poeditor-image]: https://img.shields.io/badge/POEditor-translate-green.svg
+[poeditor-url]: https://poeditor.com/join/project/1OpGjF2Jir