Update README: expand envvars section

README.adoc

@@ -936,7 +936,7 @@ The executable is at: /bin/just
 
 ==== 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 `$VARIABLE_NAME` in recipes and backticks.
+`just` will load environment variables from a file named `.env` if the setting [dotenv-load](https://github.com/casey/just#dotenv-load) is present. This file can be located in the same directory as your justfile or in a parent directory. These variables are environment variables, not `just` variables, and so must be accessed using `$VARIABLE_NAME` in recipes and backticks.
 
 For example, if your `.env` file contains:
 
@@ -1126,6 +1126,12 @@ $ just --set os bsd
 
 === Environment Variables
 
+==== How to pass environment variables to recipes
+
+There are two ways to pass environment variables to recipes: exporting `just` variables, or using an `.env` file.
+
+===== Exporting `just` variables
+
 Assignments prefixed with the `export` keyword will be exported to recipes as environment variables:
 
 ```make
@@ -1158,6 +1164,17 @@ a $A $B=`echo $A`:
   echo $A $B
 ```
 
+Note: the setting link:README.adoc#export[export] causes all `just` variables to be exported to recipes as enviroment variables.
+
+===== Using an `.env` file
+
+`just` can load an .env file if the setting link:README.adoc#dotenv-load[dotenv-load] is present. The variables in the file will be available as environment variables to the recipes. See link:README.adoc#dotenv-integration[here] for more information.
+
+==== Loading `just` variables from the shell environments variables
+
+Environment variables from the shell can be loaded as variables in `just` with the functions `env_var()` and `en_var_or_default()`, described link:README.adoc#environment-variables[here].
+They could then be exported to the recipes using link:README.adoc#exporting-just-variables[export].
+
 === Recipe Parameters
 
 Recipes may have parameters. Here recipe `build` has a parameter called `target`: