Add wiki as clonable submodule (#4588)

* Add wiki as clonable submodule per #4578

.gitmodules

@@ -31,3 +31,6 @@
 [submodule "src/3rdparty/mingw-std-threads"]
 	path = src/3rdparty/mingw-std-threads
 	url = https://github.com/meganz/mingw-std-threads.git
+[submodule "doc/wiki"]
+	path = doc/wiki
+	url = https://github.com/lmms/lmms.wiki.git

INSTALL.txt

@@ -1,16 +1,28 @@
-Building LMMS got quite simple since 0.4.0 as everything is managed
-by cmake now. Therefore make sure you have CMake (>= 2.6.0 recommended) and
-then run
+This project uses git submodules and several are required for a successful
+build.  If this tarball was NOT provided with submodules (default), some source
+dependencies will need to be manually added to src/3rdparty.
 
+If this project WAS provided with submodules, there are detailed dependencies
+(e.g. supported compilers, cmake requirements, cross-compilation) at
+doc/wiki/Compiling.md including platform-specific build instructions (e.g.
+apt-get, dnf, pacman, etc).
 
-mkdir build
-cd build
-cmake ../
-make
-sudo make install
+Online documentation:
 
-If your system does not have "sudo", become root with your preferred mechanism
-and run the "make install" command.
+   https://github.com/LMMS/lmms/wiki/
+
+Live online chat via Discord (similar to IRC):
+
+   https://lmms.io/chat, Channel #programming
+
+Basic build steps:
+
+   mkdir build
+   cd build
+   cmake ../
+   make
+   sudo make install
+   # or alternately: su -c 'make install'
 
 With the above commands an out-of-tree build is performed. You can also run
 "cmake ." directly in the root of source tree although this is not recommended.
@@ -25,21 +37,31 @@ that are going to be built into LMMS or built as plugins. Install the
 according libraries and development files if a certain feature is not enabled.
 Then remove CMakeCache.txt and run cmake again.
 
-If you want to supply an install prefix to cmake, add the flag:
+For non-root installs, or to install to a non-standard location, provide an
+install prefix to cmake, using the flag:
 
--DCMAKE_INSTALL_PREFIX=<prefix>
+   -DCMAKE_INSTALL_PREFIX=<prefix>
 
-Where <prefix> can be /usr, /usr/local, /opt, etc. The default is /usr/local.
-
-
-
-Building with QT5
-
-First of all please note that CMake >= 2.8.11 is required for building with
-Qt5 support.
+The Compilation tutorial (see docs/wiki/Compilation.md)
+specifically uses <prefix> as "../target" for all non-root installations.  This
+is strongly recommended if developing a patch or a feature.  For root
+installations, <prefix> can be /usr, /usr/local, /opt, etc. For most systems,
+the default is /usr/local.
 
 If your Qt5 installation does not reside in standard installation paths,
-additionally pass e.g.
+additionally pass:
 
--DCMAKE_PREFIX_PATH=/opt/qt53/
+   -DCMAKE_PREFIX_PATH=<prefix>
 
+Where <prefix> is the location to Qt5 installation, usually /opt/qt5/, etc.
+
+Important wiki sections:
+
+doc/wiki/Compiling.md:
+   Dependencies Detailed
+      - Build Toolchain (cmake)
+      - Compiler (gcc, clang)
+      - Libraries (required and optional dependencies)
+
+doc/wiki/Dependencies-<platform>.md
+   Platform-specific dependencies (Windows, Ubuntu, Fedora, macOS, etc.)