Add 'IgnoreDefault' option

1 files changed, 20 insertions, 2 deletions

diff --git a/README.md b/README.md
index dab2996..7f1fdca 100644
--- a/README.md
+++ b/README.md
@@ -134,10 +134,10 @@ arg.MustParse(&args)
 
 ```shell
 $ ./example -h
-Usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]] 
+Usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]]
 
 Positional arguments:
-  INPUT 
+  INPUT
   OUTPUT
 
 Options:
@@ -180,6 +180,24 @@ var args struct {
 arg.MustParse(&args)
 ```
 
+#### Ignoring environment variables and/or default values
+
+The values in an existing structure can be kept in-tact by ignoring environment
+variables and/or default values.
+
+```go
+var args struct {
+    Test  string `arg:"-t,env:TEST" default:"something"`
+}
+
+p, err := arg.NewParser(arg.Config{
+    IgnoreEnv: true,
+    IgnoreDefault: true,
+}, &args)
+
+err = p.Parse(os.Args)
+```
+
 ### Arguments with multiple values
 ```go
 var args struct {