doc: document windows shell support

explain that Git Bash and Cygwin require winpty to work correctly.
Added info on Git Bash running winpty automatically
when running node without .exe extension.

PR-URL: https://github.com/nodejs/node/pull/16104
Fixes: https://github.com/nodejs/node/issues/14100
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

BUILDING.md

@@ -59,6 +59,12 @@ note1 - The gcc4.8-libs package needs to be installed, because node
   by Joyent. SmartOS images >= 16.4 are not supported because
   GCC 4.8 runtime libraries are not available in their pkgsrc repository
 
+*Note*: On Windows, running Node.js in windows terminal emulators like `mintty`
+  requires the usage of [winpty](https://github.com/rprichard/winpty) for
+  Node's tty channels to work correctly (e.g. `winpty node.exe script.js`).
+  In "Git bash" if you call the node shell alias (`node` without the `.exe`
+  extension), `winpty` is used automatically.
+
 ### Supported toolchains
 
 Depending on host platform, the selection of toolchains may vary.