Added XML decoder (#1044)

pkg/yqlib/doc/operators/assign-update.md

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+# Assign (Update)
+
+This operator is used to update node values. It can be used in either the:
+
+### plain form: `=`
+Which will assign the LHS node values to the RHS node values. The RHS expression is run against the matching nodes in the pipeline.
+
+### relative form: `|=`
+This will do a similar thing to the plain form, however, the RHS expression is run against _the LHS nodes_. This is useful for updating values based on old values, e.g. increment.
+## Create yaml file
+Running
+```bash
+yq eval --null-input '.a.b = "cat" | .x = "frog"'
+```
+will output
+```yaml
+a:
+  b: cat
+x: frog
+```
+
+## Update node to be the child value
+Given a sample.yml file of:
+```yaml
+a:
+  b:
+    g: foof
+```
+then
+```bash
+yq eval '.a |= .b' sample.yml
+```
+will output
+```yaml
+a:
+  g: foof
+```
+
+## Double elements in an array
+Given a sample.yml file of:
+```yaml
+- 1
+- 2
+- 3
+```
+then
+```bash
+yq eval '.[] |= . * 2' sample.yml
+```
+will output
+```yaml
+- 2
+- 4
+- 6
+```
+
+## Update node from another file
+Note this will also work when the second file is a scalar (string/number)
+
+Given a sample.yml file of:
+```yaml
+a: apples
+```
+And another sample another.yml file of:
+```yaml
+b: bob
+```
+then
+```bash
+yq eval-all 'select(fileIndex==0).a = select(fileIndex==1) | select(fileIndex==0)' sample.yml another.yml
+```
+will output
+```yaml
+a:
+  b: bob
+```
+
+## Update node to be the sibling value
+Given a sample.yml file of:
+```yaml
+a:
+  b: child
+b: sibling
+```
+then
+```bash
+yq eval '.a = .b' sample.yml
+```
+will output
+```yaml
+a: sibling
+b: sibling
+```
+
+## Updated multiple paths
+Given a sample.yml file of:
+```yaml
+a: fieldA
+b: fieldB
+c: fieldC
+```
+then
+```bash
+yq eval '(.a, .c) = "potatoe"' sample.yml
+```
+will output
+```yaml
+a: potatoe
+b: fieldB
+c: potatoe
+```
+
+## Update string value
+Given a sample.yml file of:
+```yaml
+a:
+  b: apple
+```
+then
+```bash
+yq eval '.a.b = "frog"' sample.yml
+```
+will output
+```yaml
+a:
+  b: frog
+```
+
+## Update string value via |=
+Note there is no difference between `=` and `|=` when the RHS is a scalar
+
+Given a sample.yml file of:
+```yaml
+a:
+  b: apple
+```
+then
+```bash
+yq eval '.a.b |= "frog"' sample.yml
+```
+will output
+```yaml
+a:
+  b: frog
+```
+
+## Update deeply selected results
+Note that the LHS is wrapped in brackets! This is to ensure we dont first filter out the yaml and then update the snippet.
+
+Given a sample.yml file of:
+```yaml
+a:
+  b: apple
+  c: cactus
+```
+then
+```bash
+yq eval '(.a[] | select(. == "apple")) = "frog"' sample.yml
+```
+will output
+```yaml
+a:
+  b: frog
+  c: cactus
+```
+
+## Update array values
+Given a sample.yml file of:
+```yaml
+- candy
+- apple
+- sandy
+```
+then
+```bash
+yq eval '(.[] | select(. == "*andy")) = "bogs"' sample.yml
+```
+will output
+```yaml
+- bogs
+- apple
+- bogs
+```
+
+## Update empty object
+Given a sample.yml file of:
+```yaml
+{}
+```
+then
+```bash
+yq eval '.a.b |= "bogs"' sample.yml
+```
+will output
+```yaml
+{a: {b: bogs}}
+```
+
+## Update node value that has an anchor
+Anchor will remaple
+
+Given a sample.yml file of:
+```yaml
+a: &cool cat
+```
+then
+```bash
+yq eval '.a = "dog"' sample.yml
+```
+will output
+```yaml
+a: &cool dog
+```
+
+## Update empty object and array
+Given a sample.yml file of:
+```yaml
+{}
+```
+then
+```bash
+yq eval '.a.b.[0] |= "bogs"' sample.yml
+```
+will output
+```yaml
+{a: {b: [bogs]}}
+```
+