Initial readme draft (#2)

* Initial readme draft

Signed-off-by: Shane Loretz <sloretz@osrfoundation.org>

* Add USING section

Signed-off-by: Shane Loretz <sloretz@osrfoundation.org>

* Make it more clear what definitions do

Signed-off-by: Shane Loretz <sloretz@osrfoundation.org>

README.md

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+# darknet_vendor
+
+This is a CMake wrapper around [darknet](https://pjreddie.com/darknet), an open source neural network framework.
+
+# Building
+
+```
+cd darknet_vendor/
+mkdir build/
+cd build/
+cmake -DCMAKE_BUILD_TYPE=Release ..
+make
+```
+
+The package offers two CMake options to control how it is built.
+
+* `DARKNET_CUDA` (default: `On`) - Whether or not to build with CUDA support
+* `DARKNET_OPENCV` (default: `On`) - Whether or not to build with OpenCV support
+
+These can be disabled prior to compiling if you don't need either of those dependencies.
+
+```
+cmake -DDARKNET_CUDA=Off -DDARKNET_OPENCV=Off ..
+```
+
+# ROS 2 and Colcon
+
+The package includes `package.xml` and should build fine using ROS 2 Dashing.
+
+```
+colcon build --cmake-args -DCMAKE_BUILD_TYPE=Release --packages-select darknet_vendor
+```
+
+# Using darknet
+
+If you are using `ament_cmake`, use `ament_target_dependencies()` as normal.
+
+```CMake
+
+find_package(darknet_vendor REQUIRED)
+
+#...
+
+ament_target_dependencies(my_library_or_executable_target
+  darknet_vendor)
+```
+
+Otherwise, use the exported target `darknet_vendor::darknet_vendor`
+
+```CMake
+find_package(darknet_vendor REQUIRED)
+
+#...
+
+target_link_libraries(my_library_or_executable_target PUBLIC darknet_vendor::darknet_vendor)
+```
+
+Then include the `darknet.h` header in your project to make darknet functions available.
+
+```C
+#include <darknet.h>
+```
+
+Alternatively, include `darknet_vendor/darknet_vendor.h` to get preprocessor definitions with info about the installed `darknet_vendor` version.
+
+```C
+#include <darknet_vendor/darknet_vendor.h>
+```