📝 Add notes about installing python-multipart for forms (#574)

docs/tutorial/request-files.md

@@ -1,5 +1,12 @@
 You can define files to be uploaded by the client using `File`.
 
+!!! info
+    To receive uploaded files, first install [`python-multipart`](https://andrew-d.github.io/python-multipart/).
+
+    E.g. `pip install python-multipart`.
+
+    This is because uploaded files are sent as "form data".
+
 ## Import `File`
 
 Import `File` and `UploadFile` from `fastapi`:

docs/tutorial/request-forms-and-files.md

@@ -1,5 +1,10 @@
 You can define files and form fields at the same time using `File` and `Form`.
 
+!!! info
+    To receive uploaded files and/or form data, first install [`python-multipart`](https://andrew-d.github.io/python-multipart/).
+
+    E.g. `pip install python-multipart`.
+
 ## Import `File` and `Form`
 
 ```Python hl_lines="1"

docs/tutorial/request-forms.md

@@ -1,5 +1,10 @@
 When you need to receive form fields instead of JSON, you can use `Form`.
 
+!!! info
+    To use forms, first install [`python-multipart`](https://andrew-d.github.io/python-multipart/).
+
+    E.g. `pip install python-multipart`.
+
 ## Import `Form`
 
 Import `Form` from `fastapi`:

docs/tutorial/security/first-steps.md

@@ -24,6 +24,13 @@ Copy the example in a file `main.py`:
 
 ## Run it
 
+!!! info
+    First install [`python-multipart`](https://andrew-d.github.io/python-multipart/).
+
+    E.g. `pip install python-multipart`.
+
+    This is because **OAuth2** uses "form data" for sending the `username` and `password`.
+
 Run the example with:
 
 ```bash