Add docs for justfile() and justfile_directory() (#726)

README.adoc

@@ -506,6 +506,19 @@ build:
     cd {{invocation_directory()}}; ./some_script_that_needs_to_be_run_from_here
 ```
 
+==== Justfile and Justfile Directory
+
+- `justfile()` - Retrieves the path of the current justfile.
+
+- `justfile_directory()` - Retrieves the path of the parent directory of the current justfile.
+
+For example, to run a command relative to the location of the current justfile:
+
+```
+script:
+  ./{{justfile_directory()}}/scripts/some_script
+```
+
 ==== Dotenv Integration
 
 `just` will load environment variables from a file named `.env`. This file can be located in the same directory as your justfile or in a parent directory. These variables are environment variables, not `just` variables, and so must be accessed using `$VARIABLE_NAME` in recipes and backticks.