Add note to README.md about PowerShell (#69, #76)

Despite appearances, the ">" operator is actually a pipe to Out-File,
and GetFileType() returns FILE_TYPE_PIPE for that handle. It is not
seekable and does not behave like a file. By default it re-encodes its
input, which is virtually always destructive, unwanted, and surprising.
Regardless of the wording in its documentation, it is not possible to
connect process output to a file, and PowerShell does not support file
redirection.

README.md

@@ -27,8 +27,9 @@ First build the image, then run it to produce a distribution .zip file:
     docker build -t w64devkit .
     docker run --rm w64devkit >w64devkit.zip
 
-This takes about half an hour on modern systems. You will need an
-internet connection during the first couple minutes of the build.
+This takes about half an hour on modern systems. You will need an internet
+connection during the first few minutes of the build. **Note:** Do not use
+PowerShell because it lacks file redirection.
 
 ## Usage