Document how to add new custom supplements for third-party developers

For any developers who want to develop custom supplements, they need to have a src subdirectory, some C codes, and most importantly, a CMakeLists.txt file.

Here is a full list of all CMake settings that can be included in the CMakeLists.txt:
```
# SUPPL_NAME [required]
# name of the supplement package. It can be different from the directory name.
set (SUPPL_NAME newsuppl1)

# SUPPL_PROGS_SRCS [required]
# list of C source codes for supplemental modules
set (SUPPL_PROGS_SRCS psmeca.c pspolar.c pscoupe.c pssac.c)

# SUPPL_LIB_SRCS [required]
# list of C source codes for supplemental library
set (SUPPL_LIB_SRCS ${SUPPL_PROGS_SRCS} utilmeca.c sacio.c)

# SUPPL_HEADERS [optional]
# header files. Will be installed if BUILD_DEVELOPER is TRUE
set (SUPPL_HEADERS meca.h utilmeca.h sacio.h)

# SUPPL_EXAMPLE_FILES [optional]
# README and other example files
set (SUPPL_EXAMPLE_FILES README.seis)

# SUPPL_EXAMPLE_PROGS [optional]
# example scripts
set (SUPPL_EXAMPLE_PROGS test1.sh test2.sh)

# SUPPL_EXTRA_INCLUDES [optional]
# list of extra include directories
set (SUPPL_EXTRA_INCLUDES /usr/local/xxx1/include /usr/local/xxx2/include)

# SUPPL_EXTRA_LIBRARIES [optional]
# list of extra libraries to link. Can be absolute paths or library names
set (SUPPL_EXTRA_LIBS /usr/local/xxx1/lib/libxxx1.so xxx2)

# SUPPL_LIB_NAME [optional]
# name of the supplement library.
# If not set, the supplement package will be built into GMT's supplements.so.
set (SUPPL_LIB_NAME newsuppl1)

# SUPPL_DLL_RENAME [optional, Windows only]
# rename the supplement library on Windows
set (SUPPL_DLL_RENAME "newsuppl1_w32")
```

We need to document it somewhere. Maybe in the cmake/ConfigUserAdvanced.cmake or a standalone file listed in the the "Developer Resources" of the GMT documentation.