docs: add documentation for the helmignore file (#4966)

* docs: add documentation for the helmignore file
Signed-off-by: Ishaan Malhi <ishaan.malhi@gmail.com>

* docs: rearrange helmignore docs under chart template developer's guide
Signed-off-by: Ishaan Malhi <ishaan.malhi@gmail.com>

docs/README.md

@@ -24,6 +24,7 @@
   - [Variables](chart_template_guide/variables.md)
   - [Named Templates (Partials)](chart_template_guide/named_templates.md)
   - [Accessing Files Inside Templates](chart_template_guide/accessing_files.md)
+  - [Ignoring unwanted files and folders](chart_template_guide/helm_ignore_file.md)
   - [Creating a NOTES.txt File](chart_template_guide/notes_files.md)
   - [Subcharts and Global Values](chart_template_guide/subcharts_and_globals.md)
   - [Debugging Templates](chart_template_guide/debugging.md)

docs/chart_template_guide/helm_ignore_file.md

+# The .helmignore file
+
+The `.helmignore` file is used to specify files you don't want to include in your helm chart.
+
+If this file exists, the `helm package` command will ignore all the files that match the pattern specified in the `.helmignore` file while packaging your application.
+
+This can help in avoiding unncessary or sensitive files or directories from being added in your helm chart.
+
+The `.helmignore` file supports Unix shell glob matching, relative path matching, and negation (prefixed with !). Only one pattern per line is considered.
+
+Here is an example `.helmignore` file:
+
+```
+# comment
+.git
+*/temp*
+*/*/temp*
+temp?
+```
+
+**We'd love your help** making this document better. To add, correct, or remove
+information, [file an issue](https://github.com/helm/helm/issues) or
+send us a pull request.