bugfix: ensure Cursor_line is always on a text line

Manual test used here:

  abc
  ```lines
  {"p1":{"y":72,"x":82},"mode":"line","p2":{"y":29,"x":169}}
  ```
  def
  ```lines
  {"p1":{"y":36,"x":56},"mode":"line","p2":{"y":59,"x":163}}
  ```
  ```lines
  ```
  ghi
  jkl

Hitting page-down moves the cursor from abc to ghi. The 'ghi' line
should be fully visible on screen.

1 files changed, 8 insertions, 0 deletions

diff --git a/README.md b/README.md
index 7c6ddf7..240b9a6 100644
--- a/README.md
+++ b/README.md
@@ -3,3 +3,11 @@ Known issues:
   On Linux, drags using the light touch get interrupted when a key is pressed.
   You'll have to press down to drag.
 * No support yet for Unicode graphemes spanning multiple codepoints.
+* The text cursor will always stay on the screen. This can have some strange
+  implications:
+    * A long series of drawings will get silently skipped when you hit
+      page-down, until a line of text can be showed on screen.
+    * If there's no line of text at the bottom of the file, one will be
+      created.
+  So far this app isn't really designed for all-drawing files. I'm really just
+  targeting mostly-text files with a few drawings mixed in.