Update Linux build instructions

Travis CI have migrated away from running builds in Docker containers,
and no longer link to their old Docker image, so describe the
prerequisites and assumptions as accurately as possible.

README.md

@@ -28,7 +28,19 @@ Refer to `appveyor.yml` for the build process.
 
 ### Linux
 
-Refer to `.travis.yml` for the build process. A [local Docker image](https://docs.travis-ci.com/user/common-build-problems/#Troubleshooting-Locally-in-a-Docker-Image) can be used to ensure the assumed build environment.
+Refer to `.travis.yml` for the build process. It assumes that you have already
+cloned the libloot repository, that the current working directory is its root,
+and that the following applications are already installed:
+
+- `cmake`
+- `curl` and libcurl (`libcurl4-openssl-dev` on Ubuntu)
+- `git`
+- `python` and `pip` (2.7 or 3, it shouldn't matter)
+- `cargo` and the rest of the Rust toolchain (e.g. via
+  [rustup](https://rustup.rs/))
+- `wget`
+
+(The list above may be incomplete.)
 
 ### CMake Variables