README: clarify that you need to run vcvars32 first.

The source-code README file claims that you can start by just running
'cmake .'. But on Windows, that's not literally true. cmake will
expect to find a compiler on your path, and if it doesn't, will fail
to configure.

I'd always taken this for granted, assuming that anyone who was
reading this README and trying to compile PuTTY was already familiar
with how Windows compiler toolchains work, and only needed the part of
the instructions that were specific to PuTTY.

But of course there's no reason that should be true. PuTTY's primary
target audience, even from before it was called PuTTY, has been Unix
expats: people who like to do nearly everything on Unix but for some
reason have a Windows machine in front of them. Such a person might
very well be familiar with _Unix_ compilation, where the C compiler is
installed in /usr/bin and on your PATH already. And then if they want
to try to tweak PuTTY, that will be the first time they have to engage
with a Windows compiler!

So we do need to at least _say_ that the user needs to start by
running vcvars32.bat or one of its friends. Even if we don't give full
details of where to find it (because I've known it to change now and
then), we at least need to remind people to go and look for it.

Author: sgtatham

README

@@ -1,14 +1,23 @@
-This is the README for PuTTY, a free Windows and Unix Telnet and SSH
-client.
+PuTTY source code README
+========================
+
+This is the README for the source code of PuTTY, a free Windows and
+Unix Telnet and SSH client.
 
 PuTTY is built using CMake <https://cmake.org/>. To compile in the
-simplest way (on any of Linux, Windows or Mac), run these commands in
-the source directory:
+simplest way (on any of Linux, Windows or Mac), the general method is
+to run these commands in the source directory:
 
   cmake .
   cmake --build .
 
-Then, to install in the simplest way on Linux or Mac:
+These commands will expect to find a usable compile toolchain on your
+path. So if you're building on Windows with MSVC, you'll need to make
+sure that the MSVC compiler (cl.exe) is on your path, by running one
+of the 'vcvars32.bat' setup scripts provided with the tools. Then the
+cmake commands above should work.
+
+To install in the simplest way on Linux or Mac:
 
   cmake --build . --target install