Add behaves differently according to the type of the LHS:
- arrays: concatenate
- number scalars: arithmetic addition (soon)
- string scalars: concatenate (soon)

Use `+=` as append assign for things like increment. `.a += .x` is equivalent to running `.a |= . + .x`.

## Concatenate and assign arrays
Given a sample.yml file of:
```yaml
a: {val: thing, b: [cat, dog]}
'': null
```
then
```bash
yq eval '.a.b += ["cow"]' sample.yml
```
will output
```yaml
a: {val: thing, b: [cat, dog, cow]}
'': null
```

## Concatenate arrays
Given a sample.yml file of:
```yaml
a: [1, 2]
b: [3, 4]
'': null
```
then
```bash
yq eval '.a + .b' sample.yml
```
will output
```yaml
[1, 2, 3, 4]
```

## Concatenate null to array
Given a sample.yml file of:
```yaml
a: [1, 2]
'': null
```
then
```bash
yq eval '.a + null' sample.yml
```
will output
```yaml
[1, 2]
```

## Add object to array
Given a sample.yml file of:
```yaml
a: [1, 2]
c: {cat: meow}
'': null
```
then
```bash
yq eval '.a + .c' sample.yml
```
will output
```yaml
[1, 2, {cat: meow}]
```

## Add string to array
Given a sample.yml file of:
```yaml
a: [1, 2]
'': null
```
then
```bash
yq eval '.a + "hello"' sample.yml
```
will output
```yaml
[1, 2, hello]
```

## Update array (append)
Given a sample.yml file of:
```yaml
a: [1, 2]
b: [3, 4]
'': null
```
then
```bash
yq eval '.a = .a + .b' sample.yml
```
will output
```yaml
a: [1, 2, 3, 4]
b: [3, 4]
'': null
```