update gitignore and windows build instructions

.gitignore

@@ -1,4 +1,4 @@
-# exclude all dot files except .git and travis files
+# exclude all dot files except .git, appveyor and travis files
 .*
 !.git*
 !.appveyor*
@@ -10,7 +10,6 @@ bin_plugs/
 intermediate/
 intermediate_plugs/
 lib/
-src/Externals/
 *.aps
 *.user
 *.PVS-Studio.*

doc/howto/build.txt

@@ -5,9 +5,6 @@ Build:
 Currently, the only compiler supported is Visual C++ 2017. If you don't have Visual Studio yet, you can
 download and install Visual Studio 2017 Community Edition - it's absolutely free for open source projects.
 
-By default, all libraries mentioned below (except Windows SDK) are assumed to be in
-the src\Externals directory.
-
 Windows SDK is available from Visual Studio 2017 installer. You will definitely need to install the following items:
 * Windows 8.1 SDK (or newer)
 * Visual C++ MFC for x86 and x64