Added anchor operator

pkg/yqlib/doc/Anchor Operators.md

@@ -0,0 +1,29 @@
+
+## Get anchor
+Given a sample.yml file of:
+```yaml
+a: &billyBob cat
+```
+then
+```bash
+yq eval '.a | anchor' sample.yml
+```
+will output
+```yaml
+billyBob
+```
+
+## Set anchor name
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval '.a anchor = "foobar"' sample.yml
+```
+will output
+```yaml
+a: &foobar cat
+```
+