add documentation for default command

docs/commands/default.md

@@ -0,0 +1,36 @@
+# default
+
+This command sets a default row's column if missing. Other commands are capable of feeding `default` with their output through pipelines.
+
+## Usage
+```shell
+> [input-command] | default [column-name] [column-value]
+```
+
+## Examples 
+
+Let's say we have a table like this:
+
+```shell
+> open contacts.json
+━━━┯━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━
+ # │ name     │ email
+───┼──────────┼──────────────────
+ 0 │ paul     │ paul@example.com
+ 1 │ andres   │
+ 2 │ jonathan │
+━━━┷━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━
+```
+
+`default` allows us to fill `email` column with a default value:
+
+```shell
+> open contacts.json | default email "no-reply@example.com"
+━━━┯━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━
+ # │ name     │ email
+───┼──────────┼──────────────────────
+ 0 │ paul     │ paul@example.com
+ 1 │ andres   │ no-reply@example.com
+ 2 │ jonathan │ no-reply@example.com
+━━━┷━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━
+```