Fix #2184: Hyper Bootstrap! Integrate the backend as a git submodule

The backend lives in the scalac fork at
https://github.com/lampepfl/scala/tree/sharing-backend. Before this
commit, the scala-compiler built from this fork was a dependency of
Scala just to get this backend. This made it much more cumbersome to
test changes to the backend and also forced us to depend on Scala 2.11.5
since that is the version of scalac that the forked backend is based on.

This commit changes this by adding a git submodule in the
`scala-backend` directory that points to the scalac fork. We do not
compile the whole submodule, instead we add the subset of files we need
to the dotty-compiler project in the sbt build. See backend.md for more
information (online at
http://dotty.epfl.ch/docs/contributing/backend.html) once this commit is
merged.

The most important thing to note is that whenever you clone dotty you
should do `git clone --recursive` to also clone the submodule. If you
already have a cloned dotty you'll need to do:

git submodule update --init

You will need to repeat this command anytime you update dotty and the
backend has been updated, otherwise the content of `scala-backend` will
be stale.

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/docs/contributing/getting-started.md b/docs/docs/contributing/getting-started.md
index 26862e790..6f14c8299 100644
--- a/docs/docs/contributing/getting-started.md
+++ b/docs/docs/contributing/getting-started.md
@@ -24,7 +24,7 @@ Compiling and Running
 Start by cloning the repository:
 
 ```bash
-$ git clone https://github.com/lampepfl/dotty.git
+$ git clone --recursive https://github.com/lampepfl/dotty.git
 $ cd dotty
 ```