docs(installation): requires Node 8.10+

[curbengh]

• Others (Update, fix, translation, etc...)

New requirement introduced by hexojs/hexo#3778.

[SukkaW]

IMHO, maybe we can address hexojs/hexo#3789 in this PR.

| Hexo version | Required Node.js version |
|:---:|:---:|
| > 4.1.x | >= 8.10 |
| 4.0.x | >= 8.6 |
| 3.3.x - 3.9.x | >= 6.9 |
| 3.2.x - 3.3.x | >= 0.12 |
| 3.0.x - 3.1.x | >= 0.10 or iojs |
| 0.0.1 - 2.8.x | >= 0.10 |

Hexo version	Required Node.js version
> 4.1.x	>= 8.10
4.0.x	>= 8.6
3.3.x - 3.9.x	>= 6.9
3.2.x - 3.3.x	>= 0.12
3.0.x - 3.1.x	>= 0.10 or iojs
0.0.1 - 2.8.x	>= 0.10

The required Node.js version came from history of .travis.yml & package.json

[yoshinorin]

I have already commented at hexojs/hexo#3789 (comment)
I agree with add Node.js compatibility doc to website.

IMHO, should avoid using a sign of inequality (like >=, >).

My understanding, these versioning specifications are used in npm, php composer, ruby gem (I don't know any more).

But, hexo user's programming background may be various. So, I think they are possibility that they can not understand what is mean >=.