Adapted the README

Added the information from the borrower of the dockcross script.

README.md

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+cross-compilers
+===============
+
+Dockerfiles for cross compiling in a Docker container.
+
+## Features
+
+* different toolchains for cross compiling
+* commands in the container are run as the calling user, so that any created files have the expected ownership (ie. not root)
+* make variables (`CC`, `LD` etc) are set to point to the appropriate tools in the container
+* cmake and ninja are precompiled available with a toolchain file for cmake
+* current directory is mounted as the container's workdir, `/build`
+* works with boot2docker on OSX
+
+## Installation
+
+This image is not intended to be run manually. Instead, there is a helper script which comes bundled with the image.
+
+To install the helper script, run the image with no arguments, and redirect the output to a file.
+
+eg.
+```
+docker run CROSS_COMPILER_IMAGE_NAME > dockcross
+chmod +x dockcross
+mv dockcross ~/bin/
+```
+
+## Usage
+
+`dockcross [command] [args...]`
+
+Execute the given command-line inside the container.
+
+If the command matches one of the dockcross built-in commands (see below), that will be executed locally, otherwise the command is executed inside the container.
+
+---
+
+`dockcross -- [command] [args...]`
+
+To force a command to run inside the container (in case of a name clash with a built-in command), use `--` before the command.
+
+### Built-in commands
+
+`dockcross update-image`
+
+Fetch the latest version of the docker image.
+
+---
+
+`dockcross update-script`
+
+Update the installed dockcross script with the one bundled in the image.
+
+----
+
+`dockcross update`
+
+Update both the docker image, and the dockcross script.
+
+## Configuration
+
+The following command-line options and environment variables are used. In all cases, the command-line option overrides the environment variable.
+
+### DOCKCROSS_CONFIG / --config <path-to-config-file>
+
+This file is sourced if it exists.
+
+Default: `~/.dockcross`
+
+### DOCKCROSS_IMAGE / --image <docker-image-name>
+
+The docker image to run.
+
+Default: sdt4docker/raspberry-pi-cross-compiler
+
+### DOCKCROSS_ARGS / --args <docker-run-args>
+
+Extra arguments to pass to the `docker run` command.
+
+## Examples
+
+`dockcross make`
+
+Build the Makefile in the current directory.
+
+---
+
+`dockcross cmake -Bbuild -H. -GNinja`
+
+Run CMake with a build directory "build" for the CMakeLists.txt in the current directory and generate `ninja` files.
+
+`dockcross ninja -Cbuild`
+
+Run ninja in the generated build directory.
+
+---
+
+`dockcross bash -c 'find . -name \*.o | sort > objects.txt'`
+
+Note that commands are executed verbatim. If you require any shell processing for environment variable expansion or redirection, please use `bash -c 'command args...'`.
+
+---
+
+Credits go to sdt [sdt/docker-raspberry-pi-cross-compiler](https://github.com/sdt/docker-raspberry-pi-cross-compiler) on [GitHub](https://github.com), who invented the base of the `dockcross` script.