ARROW-6715: [Website] Describe "non-free" component is needed for Plasma packages in install page

Closes #86 from kou/install-plasma-non-free and squashes the following commits:

7fc190da9 <Sutou Kouhei>  Describe "non-free" component is needed for Plasma packages in install page

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>
diff --git a/install.md b/install.md
index 3feb295..48e8a3f 100644
--- a/install.md
+++ b/install.md
@@ -93,6 +93,10 @@
 sudo apt install -y -V libarrow-glib-dev # For GLib (C)
 sudo apt install -y -V libarrow-dataset-dev # For Arrow Dataset C++
 sudo apt install -y -V libarrow-flight-dev # For Flight C++
+# Notes for Plasma related packages:
+#   * You need to enable "non-free" component on Debian GNU/Linux
+#   * You need to enable "multiverse" component on Ubuntu
+#   * You can use Plasma related packages only on amd64
 sudo apt install -y -V libplasma-dev # For Plasma C++
 sudo apt install -y -V libplasma-glib-dev # For Plasma GLib (C)
 sudo apt install -y -V libgandiva-dev # For Gandiva C++