adding filename form field + updating echo package to v4 (#57)

build/docs/content/08-filename.md

@@ -0,0 +1,27 @@
+---
+title: Filename
+---
+
+All endpoints accept a form field named `filename`.
+
+If provided, the API will return the resulting PDF file with the given filename.
+Otherwise a random filename is used.
+
+> The Go and PHP libraries do not provide a way to set this form field.
+> However, you may hijack the response from the API or store the resulting PDF
+> using a custom filename.
+>
+> **Attention:** this feature does not work if a form field `webhookURL` is given.
+
+## Examples
+
+### cURL
+
+```bash
+$ curl --request POST \
+    --url http://localhost:3000/convert/html \
+    --header 'Content-Type: multipart/form-data' \
+    --form files=@index.html \
+    --form filename='result.pdf' \
+    -o result.pdf
+```