patch 8.2.4524: MS-Windows: cannot build with some sodium libraries

Problem:    MS-Windows: cannot build with some sodium libraries.
Solution:   Make the DLL name configuragle.  Add build instructions.
            (Ken Takata, closes #9905)
diff --git a/src/INSTALLpc.txt b/src/INSTALLpc.txt
index b70758c..51b315b 100644
--- a/src/INSTALLpc.txt
+++ b/src/INSTALLpc.txt
@@ -34,10 +34,11 @@
 11. Building with Ruby support
 12. Building with Tcl support
 13. Building with DirectX (DirectWrite) support
-14. Windows 3.1
-15. MS-DOS
+14. Building with libsodium support
+15. Windows 3.1
+16. MS-DOS
 
-16. Installing after building from sources
+17. Installing after building from sources
 
 
 The currently recommended way (that means it has been verified to work) is
@@ -247,9 +248,6 @@
 
     $ pacman -S git
 
-For enabling libsodium support, you also need to install the package
-
-    $ pacman -S mingw-w64-x86_64-libsodium
 
 2.3. Keep the build environment up-to-date
 
@@ -846,20 +844,55 @@
     mingw32-make -f Make_ming.mak DIRECTX=yes
 
 
-14. Windows 3.1x
+14. Building with libsodium support
+===================================
+
+For better encryption support, you can build Vim with libsodium.
+
+A) Using MSVC
+
+You can download the latest libsodium library from here:
+	https://download.libsodium.org/libsodium/releases/
+
+At this moment, libsodium-1.0.18-stable-msvc.zip is the latest package.
+Unpack it to anywhere you like, and specify the path to the SODIUM option:
+
+    nmake -f Make_mvc.mak SODIUM=C:/path/to/libsodium
+    (libsodium.dll will be used as the libsodium DLL name.)
+
+B) Using MinGW
+
+If you use msys2, you can install the libsodium package by pacman (or pacboy):
+
+    $ pacman -S mingw-w64-x86_64-libsodium	# for 64-bit Vim
+    $ pacman -S mingw-w64-i686-libsodium	# for 32-bit Vim
+    $ pacboy -S libsodium:m		# for both 32-bit and 64-bit Vim
+
+Then set SODIUM to yes:
+
+    mingw32-make -f Make_ming.mak SODIUM=yes
+    (libsodium-23.dll will be used as the libsodium DLL name.)
+
+Or you can set the path to libsodium explicitly:
+
+    mingw32-make -f Make_ming.mak SODIUM=C:/path/to/libsodium
+    (libsodium.dll will be used as the libsodium DLL name.)
+
+
+15. Windows 3.1x
 ================
 
 The Windows 3.1x support was removed in patch 7.4.1364.
 
 
-15. MS-DOS
+16. MS-DOS
 ==========
 
 The MS-DOS support was removed in patch 7.4.1399.  Only very old Vim versions
 work on MS-DOS because of the limited amount of memory available.
 
 
-16. Installing after building from sources
+17. Installing after building from sources
 ==========================================
 
 [provided by Michael Soyka, updated by Ken Takata]