doc: Documentation in Markdown for Depends Dir

Documentation more readable when viewed on Github.

Some extra changes by @laanwj:

- Make README.usage the default README. This is more convenient from a
  user perspective. Link to other documentation in this default README

- Add list of popular targets for cross compilation, change default to
  Win64 instead of Win32

depends/README.md

@@ -0,0 +1,56 @@
+### Usage
+
+To build dependencies for the current arch+OS:
+
+    make
+
+To build for another arch/OS:
+
+    make HOST=host-platform-triplet
+
+For example:
+
+    make HOST=x86_64-w64-mingw32 -j4
+
+A prefix will be generated that's suitable for plugging into Bitcoin's
+configure. In the above example, a dir named i686-w64-mingw32 will be
+created. To use it for Bitcoin:
+
+    ./configure --prefix=`pwd`/depends/x86_64-w64-mingw32
+
+Common `host-platform-triplets` for cross compilation are:
+
+- `i686-w64-mingw32` for Win32
+- `x86_64-w64-mingw32` for Win64
+- `x86_64-apple-darwin11` for MacOSX
+- `arm-linux-gnueabihf` for Linux ARM
+
+No other options are needed, the paths are automatically configured.
+
+Dependency Options:
+The following can be set when running make: make FOO=bar
+
+    SOURCES_PATH: downloaded sources will be placed here
+    BASE_CACHE: built packages will be placed here
+    SDK_PATH: Path where sdk's can be found (used by OSX)
+    FALLBACK_DOWNLOAD_PATH: If a source file can't be fetched, try here before giving up
+    NO_QT: Don't download/build/cache qt and its dependencies
+    NO_WALLET: Don't download/build/cache libs needed to enable the wallet
+    NO_UPNP: Don't download/build/cache packages needed for enabling upnp
+    DEBUG: disable some optimizations and enable more runtime checking
+
+If some packages are not built, for example `make NO_WALLET=1`, the appropriate
+options will be passed to bitcoin's configure. In this case, `--disable-wallet`.
+
+Additional targets:
+
+    download: run 'make download' to fetch all sources without building them
+    download-osx: run 'make download-osx' to fetch all sources needed for osx builds
+    download-win: run 'make download-win' to fetch all sources needed for win builds
+    download-linux: run 'make download-linux' to fetch all sources needed for linux builds
+
+### Other documentation
+
+- [description.md](description.md): General description of the depends system
+- [packages.md](packages.md): Steps for adding packages
+