docs: add CMake integration examples

NOTE: none of these have been tested, they seem sane enough

Author: ev3nvy

README.md

@@ -1,8 +1,92 @@
-SGP4 library
-============
+# SGP4 library
+## CMake support
+### External
+```cmake
+# ./CMakeLists.txt
 
-License
--------
+find_package(SGP4 1.1 CONFIG REQUIRED)
+# ...
+add_library(foo ...)
+# ...
+target_link_libraries(foo PRIVATE libsgp4::sgp4)
+# target_link_libraries(foo PRIVATE libsgp4::sgp4-static) # or static
+```
+
+### Embedded
+```cmake
+# ./CMakeLists.txt
+
+# this affects if the libsgp4::sgp4 target is shared or static
+set(SGP4_BUILD_SHARED_LIBS ON)
+# this affects if the libsgp4::sgp4-static target is available
+set(SGP4_BUILD_STATIC_LIBS ON)
+
+add_subdirectory(thirdparty/sgp4)
+# ...
+add_library(foo ...)
+# ...
+target_link_libraries(foo PRIVATE libsgp4::sgp4)
+# target_link_libraries(foo PRIVATE libsgp4::sgp4-static) # or static
+```
+
+### Embedded (FetchContent)
+```cmake
+# ./CMakeLists.txt
+
+include(FetchContent)
+
+# this affects if the libsgp4::sgp4 target is shared or static
+set(SGP4_BUILD_SHARED_LIBS ON)
+# this affects if the libsgp4::sgp4-static target is available
+set(SGP4_BUILD_STATIC_LIBS ON)
+
+FetchContent_Declare(sgp4
+  GIT_REPOSITORY https://github.com/dnwrnr/sgp4
+  # TODO: update this with an actual commit
+  GIT_TAG        bdff20d3eb6f010af95574990a662f8089db93f8 # https://github.com/dnwrnr/sgp4/tree/bdff20d3eb6f010af95574990a662f8089db93f8
+)
+FetchContent_MakeAvailable(sgp4)
+# ...
+add_library(foo ...)
+# ...
+target_link_libraries(foo PRIVATE libsgp4::sgp4)
+# target_link_libraries(foo PRIVATE libsgp4::sgp4-static) # or static
+```
+
+### External with embedded fallback
+```cmake
+# ./CMakeLists.txt
+
+# this affects if the libsgp4::sgp4 fallback target is shared or static
+set(SGP4_BUILD_SHARED_LIBS ON)
+# this affects if the libsgp4::sgp4-static fallback target is available
+set(SGP4_BUILD_STATIC_LIBS ON)
+
+# use include instead of add_subdirectory, otherwise targets won't get propagated
+include(thirdparty/CMakeLists.txt)
+add_subdirectory(src)
+```
+
+```cmake
+# ./src/CMakeLists.txt
+
+add_library(foo ...)
+# ...
+target_link_libraries(foo PRIVATE libsgp4::sgp4)
+# target_link_libraries(foo PRIVATE libsgp4::sgp4-static) # or static
+```
+
+```cmake
+# ./thirdparty/CMakeLists.txt
+
+find_package(SGP4 1.1 CONFIG)
+if(NOT SGP4_FOUND)
+  message(STATUS "Using SGP4 submodule")
+  add_subdirectory("${CMAKE_SOURCE_DIR}/thirdparty/sgp4")
+endif()
+```
+
+## License
 
     Copyright 2017 Daniel Warner