Add example and help for gum log --time option to README (#472)

* docs(log): add help for `--time` option

The `gum log --help` output for `--time` option says

```
-t, --time=""             The time format to use (kitchen, layout, ansic, rfc822, etc...)
```

with no indication of what `etc...` means. This is probably inferred for proficient Go programmers, but not for everyone else.

This commit makes it clearer which options are supported by `--time` by linking to the docs for the `time` library,

* Update README.md

* Update README.md

---------

Co-authored-by: Maas Lalani <maas@lalani.dev>

1 files changed, 5 insertions, 0 deletions

diff --git a/README.md b/README.md
index 00fa6ab..5d6b670 100644
--- a/README.md
+++ b/README.md
@@ -393,8 +393,13 @@ gum log --structured --level debug "Creating file..." name file.txt
 # Log some error.
 gum log --structured --level error "Unable to create file." name file.txt
 # ERROR Unable to create file. name=temp.txt
+
+# Include a timestamp.
+gum log --time rfc822 --level error "Unable to create file."
 ```
 
+See the Go [`time` package](https://pkg.go.dev/time#pkg-constants) for acceptable `--time` formats.
+
 See [`charmbracelet/log`](https://github.com/charmbracelet/log) for more usage.
 
 <img src="https://vhs.charm.sh/vhs-6jupuFM0s2fXiUrBE0I1vU.gif" width="600" alt="Running gum log with debug and error levels" />