add japanese translate

Hi all.

I wanna advocate this manifesto and principle of Extensible Web to Japanese web devs.

I think this thought is quite important for web future, so I wanna join this as possible as I can.
in japan, there are lots of powerful web developer, but it seem for me that they don't no about this manifesto well.
at first, I translate this in Japanese. but I don't know how publish it.

so I create this PR for asking you.

if it's works fine for you, I'm appreciate it if you publish this for example at https://ja.extensiblewebmanifesto.org/ 
or if that dosen't works. please let me to publish this at my own domain (https://jxck.io) or on my blog (http://jxck.hatenablog.com).

also, thanks for your every works for web.

thanks, and sorry for my poor english ;p

Jxck
that means "proprietary platform walls around open web" I think.

so, not "開発者を取り囲むような競合" but "オープンな Web を取り囲む競合"

so how about this ?

"In order for the open web to compete with its walled competitors, there must be a clear path for good ideas by web developers to become part of the infrastructure of the web."
"オープンな Web がそれを取りまく競合と競うためには、 Web 開発者の良いアイデアを、Web のインフラの一部とするためのはっきりとした手段が必要です。"