Windows: updated instructions

* MSYS2: Removed the reference to the update-core script, which has been removed in MSYS2: Alexpux/MSYS2-pacman#26.
* Added more details about strange prompts that might appear. :)
* Added more details on how to install Python correctly.

With these changes in place, I am successfully able to at least get the build starting. (getting non-Python related errors, but will file a separate issue about that)

README.md

@@ -80,21 +80,26 @@ Install MSYS2 from [here](https://msys2.github.io/). After you have done so, ope
 window and update the core libraries and install new packages:
 
 ```sh
-update-core
+pacman -Su
 pacman -Sy git mingw-w64-x86_64-toolchain mingw-w64-x86_64-freetype \
     mingw-w64-x86_64-icu mingw-w64-x86_64-nspr mingw-w64-x86_64-ca-certificates \
     mingw-w64-x86_64-expat mingw-w64-x86_64-cmake tar diffutils patch \
-    patchutils make python2-setuptools
-easy_install-2.7 pip virtualenv
-```
-
-Open a new MSYS shell window as Administrator and remove the Python binaries (they
-are not compatible with our `mach` driver script yet, unfortunately):
-
-```sh
-cd /mingw64/bin
-mv python2.exe python2-mingw64.exe
-mv python2.7.exe python2.7-mingw64.exe
+    patchutils make
+
+# The above might give output like below. In that case, just press Enter to use the default:
+#
+#   :: There are 16 members in group mingw-w64-x86_64-toolchain:
+#   :: Repository mingw64
+#     1) mingw-w64-x86_64-binutils  2) mingw-w64-x86_64-crt-git  3) mingw-w64-x86_64-gcc  4) mingw-w64-x86_64-gcc-ada  5) mingw-w64-x86_64-gcc-fortran
+#     6) mingw-w64-x86_64-gcc-libgfortran  7) mingw-w64-x86_64-gcc-libs  8) mingw-w64-x86_64-gcc-objc  9) mingw-w64-x86_64-gdb  10) mingw-w64-x86_64-headers-git
+#     11) mingw-w64-x86_64-libmangle-git  12) mingw-w64-x86_64-libwinpthread-git  13) mingw-w64-x86_64-make  14) mingw-w64-x86_64-pkg-config  15) mingw-w64-x86_64-tools-git
+#     16) mingw-w64-x86_64-winpthreads-git
+#
+#  Enter a selection (default=all):
+
+# Presuming the python binaries and scripts are available in the path:
+pip install -U pip setuptools
+pip install virtualenv
 ```
 
 Now, open a MINGW64 (not MSYS!) shell window, and you should be able to build servo as usual!