[simple-scan] Change recommended build and install directories

[Robert Ancell <rancell src gnome org>]

commit cdbb5723ebd0e6f7b4e1681bce075e927b4ca72b
Author: Robert Ancell <robert ancell canonical com>
Date:   Thu Dec 12 09:46:11 2019 +1300

    Change recommended build and install directories
    
    Adding the '_' prefix makes them easier to see and less likely to collide with
    a name that might otherwise be used.

 .gitignore | 4 ++--
 README.md  | 6 +++---
 2 files changed, 5 insertions(+), 5 deletions(-)
---
diff --git a/.gitignore b/.gitignore
index 3187d13..4fa760b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,2 @@
-build/*
-install/*
+_build/*
+_install/*
diff --git a/README.md b/README.md
index 4fd1efe..eade28d 100644
--- a/README.md
+++ b/README.md
@@ -29,9 +29,9 @@ $ cd simple-scan
 
 Build and run:
 ```
-$ meson --prefix $PWD/install build/
-$ ninja -C build/ all install
-$ XDG_DATA_DIRS=install/share:$XDG_DATA_DIRS ./install/bin/simple-scan
+$ meson --prefix $PWD/_install _build
+$ ninja -C _build all install
+$ XDG_DATA_DIRS=_install/share:$XDG_DATA_DIRS ./_install/bin/simple-scan
 ```
 
 # Debugging