Error for an existing source distribution

docs/err/sdist-exists.rst

@@ -0,0 +1,38 @@
+Error: Source Distribution Already Exists
+#########################################
+
+This error is presented when an attempt is made to export/create a source
+distribution of a package in a way that would overwrite an existing source
+distribution.
+
+**If exporting to a repository**, this means that a source distribution with
+the same name and version is already present in the repository. The
+``--replace`` option can be used to make ``dds`` forcibly overwrite the source
+distribution in the repository. This will be a common workflow when developing
+a package and one desires to see those changes reflected in another project
+that is try to use it.
+
+**If creating a source distribution manually**, this means that the destination
+path of the source distribution directory is already an existing directory
+(which may not be a source distribution itself). If ``dds`` were to try and
+write a source distribution to the named path, it would be required to delete
+whatever exists there before creating the source distribution.
+
+.. warning::
+    When using ``dds sdist create`` with the ``--out <path>`` parameter, the
+    ``<path>`` given **is not the directory in which to place the source
+    distribution, but the filepath to the source distribution itself**!
+
+    If I have a directory named ``foo/``, and I want to create a source
+    distribution in that directory, **the following command is incorrect**::
+
+        # Do not do this:
+        dds sdist create --out foo/
+
+    If you pass ``--replace`` to the above command, ``dds`` will **destroy the
+    existing directory** and replace it with the source distribution!
+
+    You **must** provide the full path to the source distribution::
+
+        # Do this:
+        dds sdist create --out foo/my-project.dsd