maketgz: put docs/RELEASE-TOOLS.md into the tarball

[bagder]

Generated with scripts/release-tools.sh

The script lists the exact Debian package names and version numbers for the tools that are used to generate the tarball.

docs/RELEASE-TOOLS.md

# Release tools

The following tools and their Debian package version numbers were used to
produce this release tarball.

- autoconf: 2.71-3
- automake: 1:1.16.5-1.3
- libtool: 2.4.7-7
- make: 4.3-4.1
- perl: 5.38.2-3.2
- git: 1:2.43.0-1+b1

# Reproduce the tarball

- Clone the repo and checkout the release tag
- Install the same set of tools + versions as listed above

## Do a standard build

- autoreconf -fi
- ./configure [...]
- make

## Generate the tarball

- ./maketgz [version]

[bagder]

The script also uses sh, find, date, sed etc but I cannot see that their versions should be relevant