Add plugin development documentation

docs/plugin_development.md

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+# Writing storage plugins
+
+There are different interfaces for storage plugins depending on your need: The general `ReadWriteStorage` and the more specific `ReadableStorage`.
+
+## Writing a general plugin
+
+Assume you write a plugin `MyStorage` which can both save messages and read messages.
+Its header file could be `my_storage.hpp` and it would derive from `rosbag2_storage::ReadWriteStorage`.
+While implementing the interface provided by `rosbag2_storage::ReadWriteStorage`, make sure that all resources such as file handles or database connections are closed or destroyed in the destructor, no additional `close` call should be necessary.
+
+In order to find the plugin at runtime, it needs to be exported to the pluginlib.
+Add the following lines to `my_storage.cpp`:
+
+```
+#include "pluginlib/class_list_macros.hpp"
+PLUGINLIB_EXPORT_CLASS(MyStorage, rosbag2_storage::ReadWriteStorage)
+```
+
+Furthermore, we need some meta-information in the form of a `plugin_description.xml` file.
+Here, it contains
+
+```
+<library path="my_storage_lib">
+  <class name="my_storage" type="MyStorage" base_class_type="rosbag2_storage::ReadWriteStorage">
+  </class>
+</library>
+```
+Here, `my_storage_lib` is the name of the library while `my_storage` is an identifier used by the pluginlib to load it.
+
+In addition, in the `CMakeLists.txt` plugins and `plugin_description` file need to be added to the index to be found at runtime:
+
+`pluginlib_export_plugin_description_file(rosbag2_storage plugin_description.xml)`
+
+The first argument `rosbag2_storage` denotes the library we add our plugin to, while the second argument is the path to the plugin description file.
+
+## Writing a plugin for reading only
+
+When writing plugins to only provide functionality for reading, derive from `rosbag2_storage::ReadableStorage`.
+
+If the read-only plugin is called `my_readonly_storage` in a library `my_storage_lib`, it will be registered using 
+
+```
+#include "pluginlib/class_list_macros.hpp"
+PLUGINLIB_EXPORT_CLASS(MyReadonlyStorage, rosbag2_storage::ReadableStorage)
+```
+with the plugin description
+```
+<library path="my_storage_lib">
+  <class name="my_readonly_storage" type="MyReadonlyStorage" base_class_type="rosbag2_storage::ReadableStorage">
+  </class>
+</library>
+```
+and the usual pluginlib export in the CMakeLists:
+
+`pluginlib_export_plugin_description_file(rosbag2_storage plugin_description.xml)`