Added unix installation instructions

INSTALL.md

@@ -10,3 +10,20 @@
 6. Add `%ADR_HOME%\launch-scripts\adr.bat` to the `PATH` environment variable
 
 You should now be able to type `adr` from the command line and see a response.
+
+## Unix
+
+1. Download the source code.
+2. Make sure you have gradle installed (https://gradle.org/).
+3. Run `.\gradlew releaseJar`. This should create a file `build/releases/adr-j.jar`.
+4. Set the environment variable `ADR_HOME` to the folder where you downloaded the source code. This should contain the `build` folder. For instance this could be done by using the adding the following to the `~/.bashrc` file:
+```
+export ADR_HOME=~/adr-j
+```
+
+5. Set the environment variable `EDITOR` or `VISUAL` to the location of the editor you what to use for editing the ADRs (e.g. Atom), e.g in the `~/.bashrc` file:
+```
+export EDITOR=/usr/bin/vi
+```
+
+6. Move `%ADR_HOME%\launch-scripts\adr` to the `~/bin` directory.