docs: add docs for creating a GoogleDriveFile by ID

docs/file.rst

@@ -0,0 +1,36 @@
+File referencing made easy
+=============================
+
+*PyDrive* can create a single `GoogleDriveFile`_ if you already know
+the file's ID.
+
+Specify a file by ID
+--------------------
+
+Create a `GoogleDriveFile`_ instance with a ``dict`` specifying the
+Google Drive file ID.  Then call the various ``GetContent*()`` methods
+(i.e., `GetContentString()`_, `GetContentFile()`_, and
+`GetContentIOBuffer()`_) to download contents of that file.
+
+.. code-block:: python
+
+    import mimetypes
+    from pydrive2.files import GoogleDriveFile
+
+    # Download a file as a PPTX (e.g., a Google Slides file, or even
+    # an actual PPTX file)
+    def download_pptx(id, output_file='my-slides.pptx'):
+        mtype = mimetypes.guess_type(output_file)[0]
+        gfile = GoogleDriveFile(gauth, {'id': id})
+        gfile.GetContentFile(filename=output_file, mimetype=mtype)
+
+    # Download a text file as a string
+    def download_text_file(id):
+        mtype = 'text/plain'
+        gfile = GoogleDriveFile(gauth, {'id': id})
+        return gfile.GetContentString(mimetype=mtype)
+
+.. _`GoogleDriveFile`: /PyDrive2/pydrive2/#pydrive2.files.GoogleDriveFile
+.. _`GetContentFile()`: /PyDrive2/pydrive2/#pydrive2.files.GetContentFile
+.. _`GetContentString()`: /PyDrive2/pydrive2/#pydrive2.files.GetContentString
+.. _`GetContentIOBuffer()`: /PyDrive2/pydrive2/#pydrive2.files.GetContentIOBuffer

docs/index.rst

@@ -44,6 +44,7 @@ Table of Contents
    oauth
    filemanagement
    filelist
+   file
    fsspec
    pydrive2
    genindex