.gitignore with copyright using better search for copyright notice

In the previous commit, a copyright notice was added after a systematic
search of the version controlled files. However, we missed `.gitignore'
(because we were discarding those with the `*.git*' pattern to avoid files
in the `.git' directory). This has been fixed by using this command (in the
top project directory) instead:

    for f in $(find ./ -type f); do \
      if [[ $f != ./.git/* ]]; then \
        n=$(grep -i copyright $f | wc -l); \
        echo "$n $f"; \
      fi; \
    done | awk '$1==0'

2 files changed, 16 insertions, 0 deletions

diff --git a/.file-metadata b/.file-metadata
index 062e1f1..9dd95e7 100644
--- a/.file-metadata
+++ b/.file-metadata
diff --git a/.gitignore b/.gitignore
index 5521ca7..e6dc938 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,19 @@
+# Files to ignore in version control (Git) -*- conf-unix -*-
+# ==========================================================
+#
+# The files satisfying the patterns here (one per line) are ignored by
+# the version control using Git, they are all outputs of the various
+# tools used to built Gnuastro, so we don't need to keep their
+# history.
+#
+# To help readability, please add new files based on the length of the
+# line you wish to add.
+#
+# Copying and distribution of this file, with or without modification,
+# are permitted in any medium without royalty provided the copyright
+# notice and this notice are preserved.  This file is offered as-is,
+# without any warranty.
+
 *~
 *.txt
 *.aux