README.md: Additional information for Windows Installation.

[freakse-sa]

Proposal

How about to show the dafault path for windows user ?

Rationale

For the windows users, installer sets the path. But non-technical user may not check it. and for them, it can be difficult to specify the path.
(Evenif they can search at Google search how to set environmental variable.)

How about this message ?

Windows Installation:

1. Download bbl-1.4.msi, choose where to install, and click Install button,.
2. Add folder (The default is "C:\Users\YOUR-USERNAME\AppData\Local\bbl") where you put bbl.exe to your %PATH% by editing environmental variable setting, thenbbl command should be available.