Add recommendation for 2GB RAM

We noticed on multiple places that machines with less than 2GB of RAM
fail their build and result in missing files and unexpected errors.

Sadly we can't really solve this right now since it's a webpack
related bug.

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

Author: SISheogorath

README.md

@@ -57,6 +57,7 @@ Thanks for using! :smile:
 - Node.js 6.x or up (test up to 7.5.0)
 - 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
 
 ### Instructions