#324 implement dotenv

docs/usage.md

@@ -33,8 +33,10 @@ executable called must be available by the OS or in PATH.
 
 If you omit a task name, "default" will be assumed.
 
-## Environment
+## Environment variables
 
+
+### Task
 You can use `env` to set custom environment variables for a specific task:
 
 ```yaml
@@ -66,6 +68,39 @@ tasks:
 > NOTE: `env` supports expansion and retrieving output from a shell command
 > just like variables, as you can see on the [Variables](#variables) section.
 
+
+### Operating System
+Environment variables from the OS are accessible using `$VARNAME`:
+
+```yaml
+version: '2'
+
+tasks:
+  greet:
+    cmds:
+      - echo "Hello $USER"
+```
+
+### .env
+
+*.env* files are supported in v3 using the `dotenv` declaration:
+
+.env
+```
+KEYNAME=VALUE
+```
+Taskfile.yml
+```yaml
+version: '3'
+
+dotenv: ['.env']
+
+tasks:
+  greet:
+    cmds:
+      - echo "Using $KEYNAME"
+```
+
 ## Operating System specific tasks
 
 If you add a `Taskfile_{{GOOS}}.yml` you can override or amend your Taskfile