Renamed XPI build script, added info about it to readme.

.gitignore

@@ -1,2 +1,2 @@
 /build/
-*.zip
+*.xpi

README.adoc

@@ -111,10 +111,14 @@ You can run the tests with `ctest` inside the build directory. Install with
 
 == Browser plugins
 
+=== WebExtension
+
 The
 https://schlomp.space/tastytea/remwharead/src/branch/main/browser-plugins/webextension[WebExtension]
 works in Firefox and possibly other browsers with WebExtension support. You
-can install it from https://addons.mozilla.org/en-US/firefox/addon/remwharead/[addons.mozilla.org].
+can install it from
+https://addons.mozilla.org/en-US/firefox/addon/remwharead/[addons.mozilla.org]
+or build it yourself with `build_xpi.sh`.
 
 == Copyright
 

browser-plugins/webextension/build_package.sh

@@ -1,3 +0,0 @@
-#!/bin/sh
-
-zip -r -FS ../remwharead.zip * --exclude 'native-wrapper/*' --exclude build_package.sh

browser-plugins/webextension/build_xpi.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+zip -r -FS ../remwharead.xpi --exclude 'native-wrapper/*' --exclude build_xpi.sh -- *