Update docs for Go 1.11

CONTRIBUTING.md

@@ -46,12 +46,15 @@ Remember, the easier it is for us to reproduce the bug, the earlier it will be
 corrected!
 
 In addition, you can compile restic with debug support by running
-`go run build.go -tags debug` and instructing it to create a debug log by
-setting the environment variable `DEBUG_LOG` to a file, e.g. like this:
+`go run -mod=vendor build.go -tags debug` and instructing it to create a debug
+log by setting the environment variable `DEBUG_LOG` to a file, e.g. like this:
 
     $ export DEBUG_LOG=/tmp/restic-debug.log
     $ restic backup ~/work
 
+For Go < 1.11, you need to remove the `-mod=vendor` option from the build
+command.
+
 Please be aware that the debug log file will contain potentially sensitive
 things like file and directory names, so please either redact it before
 uploading it somewhere or post only the parts that are really relevant.
@@ -60,9 +63,37 @@ uploading it somewhere or post only the parts that are really relevant.
 Development Environment
 =======================
 
-In order to compile restic with the `go` tool directly, it needs to be checked
-out at the right path within a `GOPATH`. The concept of a `GOPATH` is explained
-in ["How to write Go code"](https://golang.org/doc/code.html).
+The repository contains several sets of directories with code: `cmd/` and
+`internal/` contain the code written for restic, whereas `vendor/` contains
+copies of libraries restic depends on. The libraries are managed with the
+command `go mod vendor`.
+
+Go >= 1.11
+----------
+
+For Go version 1.11 or later, you should clone the repo (without having
+`$GOPATH` set) and `cd` into the directory:
+
+    $ unset GOPATH
+    $ git clone https://github.com/restic/restic
+    $ cd restic
+
+Then use the `go` tool to build restic:
+
+    $ go build ./cmd/restic
+    $ ./restic version
+    restic 0.9.2-dev (compiled manually) compiled with go1.11 on linux/amd64
+
+You can run all tests with the following command:
+
+    $ go test ./...
+
+Go < 1.11
+---------
+
+In order to compile restic with Go before 1.11, it needs to be checked out at
+the right path within a `GOPATH`. The concept of a `GOPATH` is explained in
+["How to write Go code"](https://golang.org/doc/code.html).
 
 If you do not have a directory with Go code yet, executing the following
 instructions in your shell will create one for you and check out the restic
@@ -83,12 +114,7 @@ You can then build restic as follows:
 
 The following commands can be used to run all the tests:
 
-    $ go test ./cmd/... ./internal/...
-
-The repository contains two sets of directories with code: `cmd/` and
-`internal/` contain the code written for restic, whereas `vendor/` contains
-copies of libraries restic depends on. The libraries are managed with the
-[`dep`](https://github.com/golang/dep) tool.
+    $ go test ./...
 
 Providing Patches
 =================