doc/signup.md: update Writers before upspin-ui

Upspin-ui does not deal well with putting the servers
into config before the new user has write permission
on the servers. Perhaps that can be improved, but first
let's fix the doc.

Change-Id: I96da523ee722b9bd6464eae2bf73faa1ddeafcc1
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19401
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/doc/signup.md b/doc/signup.md
index 5a3b131..52b0e1e 100644
--- a/doc/signup.md
+++ b/doc/signup.md
@@ -61,6 +61,13 @@
 > Conversely, if you lose your keys your email account is not useful for
 > recovery.
 
+If you are planning to join an existing Upspin directory and store server,
+before starting the next step with `upspin-ui`,
+make sure to ask the administrator to add your user name to the server's
+`Writers` group. (They'll know what to do.)
+This will grant you permission to create your user root in that directory
+server and store data in that store server.
+
 **Start the `upspin-ui` program to start the signup process.**
 
 The first step asks for your user name, generates a key pair (one secret key,
@@ -135,16 +142,10 @@
 
 ## Creating your Upspin directory {#mkdir}
 
-If you choose to use existing servers or to deploy your own, the `upspin-ui`
+Whether you chose to use existing servers or to deploy your own, the `upspin-ui`
 program will attempt to create a directory in the nominated directory server
 that will host your Upspin tree (your "user root").
 
-> If you are planning to join an existing Upspin directory and store server,
-> make sure to ask the administrator to add your user name to the server's
-> `Writers` group. (They'll know what to do.)
-> This will grant you permission to create your user root in that directory
-> server and store data in that store server.
-
 With the servers running and granting you access permission, and with your
 correct information registered in the key server, `upspin-ui` will create
 your user root and display its contents.