Added XML decoder (#1044)

pkg/yqlib/doc/operators/equals.md

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+# Equals
+
+This is a boolean operator that will return `true` if the LHS is equal to the RHS and `false` otherwise.
+
+```
+.a == .b
+```
+
+It is most often used with the select operator to find particular nodes:
+
+```
+select(.a == .b)
+```
+
+## Match string
+Given a sample.yml file of:
+```yaml
+- cat
+- goat
+- dog
+```
+then
+```bash
+yq eval '.[] | (. == "*at")' sample.yml
+```
+will output
+```yaml
+true
+true
+false
+```
+
+## Don't match string
+Given a sample.yml file of:
+```yaml
+- cat
+- goat
+- dog
+```
+then
+```bash
+yq eval '.[] | (. != "*at")' sample.yml
+```
+will output
+```yaml
+false
+false
+true
+```
+
+## Match number
+Given a sample.yml file of:
+```yaml
+- 3
+- 4
+- 5
+```
+then
+```bash
+yq eval '.[] | (. == 4)' sample.yml
+```
+will output
+```yaml
+false
+true
+false
+```
+
+## Dont match number
+Given a sample.yml file of:
+```yaml
+- 3
+- 4
+- 5
+```
+then
+```bash
+yq eval '.[] | (. != 4)' sample.yml
+```
+will output
+```yaml
+true
+false
+true
+```
+
+## Match nulls
+Running
+```bash
+yq eval --null-input 'null == ~'
+```
+will output
+```yaml
+true
+```
+
+## Non exisitant key doesn't equal a value
+Given a sample.yml file of:
+```yaml
+a: frog
+```
+then
+```bash
+yq eval 'select(.b != "thing")' sample.yml
+```
+will output
+```yaml
+a: frog
+```
+
+## Two non existent keys are equal
+Given a sample.yml file of:
+```yaml
+a: frog
+```
+then
+```bash
+yq eval 'select(.b == .c)' sample.yml
+```
+will output
+```yaml
+a: frog
+```
+