liblammps.pc: add some more documentation

cmake/pkgconfig/liblammps.pc.in

@@ -1,9 +1,29 @@
 # pkg-config file for lammps
 # https://people.freedesktop.org/~dbn/pkg-config-guide.html
-# Usage: cc `pkg-config --cflags --libs liblammps` -o myapp myapp.c
+
-# after you added @CMAKE_INSTALL_FULL_LIBDIR@/pkg-config to PKG_CONFIG_PATH,
+# Add the directory where lammps.pc got installed to your PKG_CONFIG_PATH
 # e.g. export PKG_CONFIG_PATH=@CMAKE_INSTALL_FULL_LIBDIR@/pkgconfig
 
+# Use this on commandline with:
+# c++ `pkg-config --cflags --libs lammps` -o myapp myapp.cpp
+
+# Use this in a Makefile:
+# myapp: myapp.cpp
+# 	$(CC) `pkg-config --cflags --libs lammps` -o $@ $<
+
+# Use this in autotools:
+# configure.ac:
+# PKG_CHECK_MODULES([LAMMPS], [lammps])
+# Makefile.am:
+# myapp_CFLAGS = $(LAMMPS_CFLAGS)
+# myapp_LDADD = $(LAMMPS_LIBS)
+
+# Use this in CMake:
+# CMakeLists.txt:
+# find_package(PkgConfig)
+# pkg_check_modules(LAMMPS IMPORTED_TARGET lammps)
+# target_link_libraries(<lib> PkgConfig::LAMMPS)
+
 prefix=@CMAKE_INSTALL_PREFIX@
 libdir=@CMAKE_INSTALL_FULL_LIBDIR@
 includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@