Added base64 support

pkg/yqlib/doc/usage/base64.md

@@ -0,0 +1,23 @@
+
+{% hint style="warning" %}
+Note that versions prior to 4.18 require the 'eval/e' command to be specified. 
+
+`yq e <exp> <file>`
+{% endhint %}
+
+## Decode Base64
+Decoded data is assumed to be a string.
+
+Given a sample.yml file of:
+```yml
+V29ya3Mgd2l0aCBVVEYtMTYg8J+Yig==
+```
+then
+```bash
+yq -p=props sample.properties
+```
+will output
+```yaml
+V29ya3Mgd2l0aCBVVEYtMTYg8J+Yig: =
+```
+