Task: Add merge command docs

Resolves: #35

mkdocs/merge.md

@@ -0,0 +1,104 @@
+Yaml files can be merged using the 'merge' command. Each additional file merged with the first file will
+set values for any key not existing already or where the key has no value.
+
+```
+yaml m <yaml_file|json_file> <path>...
+```
+{!snippets/works_with_json.md!}
+
+### To Stdout
+Given a data1.yaml file of:
+```yaml
+a: simple
+b: [1, 2]
+```
+and data2.yaml file of:
+```yaml
+a: other
+c:
+  test: 1
+```
+then
+```bash
+yaml m data1.yaml data2.yaml
+```
+will output:
+```yaml
+a: simple
+b: [1, 2]
+c:
+  test: 1
+```
+
+### Updating files in-place
+Given a data1.yaml file of:
+```yaml
+a: simple
+b: [1, 2]
+```
+and data2.yaml file of:
+```yaml
+a: other
+c:
+  test: 1
+```
+then
+```bash
+yaml m -i data1.yaml data2.yaml
+```
+will update the data1.yaml file so that the value of 'c' is 'test: 1'.
+
+### Overwrite values
+Given a data1.yaml file of:
+```yaml
+a: simple
+b: [1, 2]
+```
+and data2.yaml file of:
+```yaml
+a: other
+c:
+  test: 1
+```
+then
+```bash
+yaml m -x data1.yaml data2.yaml
+```
+will output:
+```yaml
+a: other
+b: [1, 2]
+c:
+  test: 1
+```
+
+### Overwrite values with arrays
+Given a data1.yaml file of:
+```yaml
+a: simple
+b: [1, 2]
+```
+and data3.yaml file of:
+```yaml
+b: [2, 3, 4]
+c:
+  test: 2
+  other: true
+d: false
+```
+then
+```bash
+yaml m -x data1.yaml data3.yaml
+```
+will output:
+```yaml
+a: simple
+b: [2, 3, 4]
+c:
+  test: 2
+  other: true
+d: false
+```
+
+Notice that 'b' does not result in the merging of the values within an array. The underlying library does not
+currently handle merging values within an array.