Add some SPDX headers.

INSTALL.adoc

@@ -1,4 +1,6 @@
 = Installing Open Adventure =
+// SPDX-FileCopyrightText: Eric S. Raymond <esr@thyrsus.com>
+// SPDX-License-Identifier: CC-BY-4.0
 
 Installation now requires Python3 due to a security issue
 with the YAML library.

README.adoc

@@ -1,4 +1,6 @@
 = README for Open Adventure =
+// SPDX-FileCopyrightText: Eric S. Raymond <esr@thyrsus.com>
+// SPDX-License-Identifier: CC-BY-4.0
 
 If you are reading this anywhere but at http://www.catb.org/~esr/open-adventure
 you can go there for tarball downloads and other resources.

advent.adoc

@@ -1,5 +1,7 @@
 = advent(6) =
 :doctype: manpage
+// SPDX-FileCopyrightText: Eric S. Raymond <esr@thyrsus.com>
+// SPDX-License-Identifier: CC-BY-4.0
 
 == NAME ==
 advent - Colossal Cave Adventure

hints.adoc

@@ -1,4 +1,6 @@
 = Non-spoiler hints =
+// SPDX-FileCopyrightText: Eric S. Raymond <esr@thyrsus.com>
+// SPDX-License-Identifier: CC-BY-4.0
 
 Say the words you see.  They can have interesting effects.
 

history.adoc

@@ -1,5 +1,7 @@
 = A brief history of Colossal Cave Adventure =
 by Eric S. Raymond
+// SPDX-FileCopyrightText: Eric S. Raymond <esr@thyrsus.com>
+// SPDX-License-Identifier: CC-BY-4.0
 
 Adventure is the fons et origo of all later dungeon-crawling computer
 games, the granddaddy of interactive fiction, and one of the hallowed

notes.adoc

@@ -1,5 +1,7 @@
 = Open Adventure Maintainer's Notes =
 by Eric S. Raymond
+// SPDX-FileCopyrightText: Eric S. Raymond <esr@thyrsus.com>
+// SPDX-License-Identifier: CC-BY-4.0
 
 In which we explain what has been done to this code since Don Woods
 authorized us to ship it under an open-source license.  There's a