Add new question about Snowflake

In Censorship section, add new question about our new pluggable transport Snowflake:

• What is Snowflake
• How it works (briefly) and benefits
• Platforms supported (Windows, macOS, Linux; Android is a working in progress)
• How you can use it with Tor Browser Alpha
• How to run snowflakes proxies to help other users
• Say it's under development

Resources:

• https://snowflake.torproject.org/
• https://trac.torproject.org/projects/tor/wiki/doc/Snowflake