Fix yarn install guide for typescript

[raymondsze]

Should be yarn global add typescript?

[coveralls]

Coverage remained the same at 89.74% when pulling cb7fca5 on raymondsze:master into 455e8ac on tngan:master.

[tngan]

@raymondsze My bad, the usage of package manager should be consistent, the second command is correct for installing dependencies.

$ yarn global add typescript
$ yarn

Can you help to modify it then I will accept the PR ? Thanks a lot.

[coveralls]

Coverage remained the same at 89.74% when pulling d8acf59 on raymondsze:master into 455e8ac on tngan:master.