Start documentation for filesystems

We have quite a few GSoC projects around this, it would make sense to
have some documentation for people attempting them.

Add some generic overview of filesystems and how to test them.

docs/develop/file_systems/overview.txt

@@ -0,0 +1,55 @@
+Writing filesystem drivers for Haiku
+====================================
+
+Filesystem drivers are in src/add-ons/kernel/file_system
+
+A filesystem usually relies on an underlying block device, but that's not
+required. For example, NFS is a network filesystem, so it doesn't need one.
+
+Development tools
+-----------------
+
+# fs_shell
+
+It is not convenient to test a filesystem by reloading its driver into a
+running Haiku system (kernel debugging is often not as easy as userland).
+Moreover, the filesystem interacts with other components of the system
+(file cache, block cache, but also any application reading or writing files).
+
+For the early development steps, it is much easier to run the filesystem code
+in a more controlled environment. This can be achieved through the use of
+a "filesystem shell": a simple application that runs the filesystem code, and
+allows performing specific operations through a command line interface.
+
+Example of fs_shell implementations are available under src/tests/add-ons/kernel/file_systems/
+for the bfs and btrfs filesystems.
+
+For example, to build the fs_shell for btrfs, use
+
+   jam -q "<build>btrfs_shell"
+
+To run it, use
+
+   jam run objects/haiku_host/x86_gcc2/release/tests/add-ons/kernel/file_systems/btrfs/btrfs_shell/btrfs_shell [arguments]
+
+You need to pass at least a file or device containing a filesystem image as an
+argument. You need some tool to create one. It is possible to work using an
+actual disk volume (but be careful, it's risky to use one with useful data in it),
+a file, or a RAM disk, depending on what you are doing.
+
+# userlandfs
+
+As a second step, it's possible to use the filesystem as part of a runing
+system, while still running it in userland. This allows use of Debugger,
+memory protection, and in general any kind of userland debugging or tracing
+tool. When the filesystem crashes, it does not bring down the whole system.
+
+Userlandfs can run the filesystem code using the same interface as the kernel,
+therefore, once everything is working with userlandfs, running the filesystem
+as kernel code is usually quite easy (and provides a performance boost)
+
+# Torture and performance tests
+
+Once the basic operations are working fine, it is a good idea to perform more
+agressive testing. Examples of scripts doing this are available in
+src/tests/add-ons/kernel/file_systems/ for the fat and ext2 filesystems.