Add instructions to README.txt for using git

.absolute_gitmodules

@@ -0,0 +1,27 @@
+[submodule "Resources/Binary"]
+	path = Resources/Binary
+	url = https://github.com/OoliteProject/oolite-binary-resources.git
+[submodule "Mac-specific"]
+	path = Mac-specific
+	url = https://github.com/OoliteProject/oolite-mac-components
+[submodule "deps/Linux-deps"]
+	path = deps/Linux-deps
+	url = https://github.com/OoliteProject/oolite-linux-dependencies.git
+[submodule "tests"]
+	path = tests
+	url = https://github.com/OoliteProject/oolite-tests.git
+[submodule "deps/Cross-platform-deps"]
+	path = deps/Cross-platform-deps
+	url = https://github.com/OoliteProject/oolite-sdl-dependencies.git
+[submodule "deps/mozilla"]
+	path = deps/mozilla
+	url = https://github.com/OoliteProject/spidermonkey-ff4.git
+[submodule "deps/libogg"]
+	path = deps/libogg
+	url = https://github.com/OoliteProject/libogg-1.3.0.git
+[submodule "deps/libvorbis"]
+	path = deps/libvorbis
+	url = https://github.com/OoliteProject/libvorbis-1.3.3.git
+[submodule "deps/Windows-deps"]
+	path = deps/Windows-deps
+	url = https://github.com/OoliteProject/oolite-windows-dependencies.git

.relative_gitmodules

@@ -0,0 +1,27 @@
+[submodule "Resources/Binary"]
+	path = Resources/Binary
+	url = ../oolite-binary-resources.git
+[submodule "Mac-specific"]
+	path = Mac-specific
+	url = ../oolite-mac-components
+[submodule "deps/Linux-deps"]
+	path = deps/Linux-deps
+	url = ../oolite-linux-dependencies.git
+[submodule "tests"]
+	path = tests
+	url = ../oolite-tests.git
+[submodule "deps/Cross-platform-deps"]
+	path = deps/Cross-platform-deps
+	url = ../oolite-sdl-dependencies.git
+[submodule "deps/mozilla"]
+	path = deps/mozilla
+	url = ../spidermonkey-ff4.git
+[submodule "deps/libogg"]
+	path = deps/libogg
+	url = ../libogg-1.3.0.git
+[submodule "deps/libvorbis"]
+	path = deps/libvorbis
+	url = ../libvorbis-1.3.3.git
+[submodule "deps/Windows-deps"]
+	path = deps/Windows-deps
+	url = ../oolite-windows-dependencies.git

README.txt

@@ -78,3 +78,28 @@ deposited in the top level.
 On OS X, you can run from Xcode by clicking on the appropriate icon
 (or choosing 'Build and Run').
 On Linux/BSD/Unix, in a terminal, type 'openapp oolite'
+
+5. Git
+------
+
+The Oolite source is available from github.  Use
+
+ git clone https://OoliteProject/oolite
+
+to retrieve.  Then
+
+ git submodule update --init
+
+to fetch the various submodules.
+
+If you've cloned the source from a forked repository instead, this may
+not work - due to relative directory paths in .gitmodules, git tries
+to download the submodules from the fork instead of the original oolite
+repository.  A workaround is to copy the file .absolute_gitmodules
+onto .gitmodules, then perform the submodules init, then replace
+.gitmodules with .relative_submodules.  eg, on Unix:
+
+$ cp .absolute_gitmodules .gitmodules
+$ git submodule upate --init
+$ cp .relative_gitmodules .gitmodules
+