Add README documenting how to use hackage-repo-tool

This has been shamelessly stolen with some modifications from the
blogpost

https://www.well-typed.com/blog/2015/07/hackage-security-alpha/

Fixes #173

hackage-repo-tool/README.md

@@ -0,0 +1,96 @@
+# `hackage-repo-tool`: Manage secure file-based package repositories
+
+**Please refer to the [package description](https://hackage.haskell.org/package/hackage-repo-tool#description) for an overview of `hackage-repo-tool`, TUF and `hackage-security`.**
+
+## Setting up a secure file-based repo
+
+A file-based repository (as opposed to one running the actual [Hackage software](https://hackage.haskell.org/package/hackage-server)) is much easier to set up and will suffice for many purposes.  Note that such a local file-based package repository can be turned into a remotely accessible secure package repository by any HTTP server supporting static file serving such as Nginx or Apache httpd.
+
+1.  Create a directory `~/my-secure-repo` containing a single
+    subdirectory `~/my-secure-repo/package`. Put whatever packages you
+    want to make available from your repo in this subdirectory. At this
+    point your repository might look like
+
+        ~/my-secure-repo/package/basic-sop-0.1.0.5.tar.gz
+        ~/my-secure-repo/package/generics-sop-0.1.1.1.tar.gz
+        ~/my-secure-repo/package/generics-sop-0.1.1.2.tar.gz
+        ~/my-secure-repo/package/json-sop-0.1.0.4.tar.gz
+        ~/my-secure-repo/package/lens-sop-0.1.0.2.tar.gz
+        ~/my-secure-repo/package/pretty-sop-0.1.0.1.tar.gz
+        ~/my-secure-repo/package/HsYAML-0.2.1.0.tar.gz
+
+    Note the flat directory structure: different packages and
+    different versions of those packages all live in the one
+    directory.
+
+2.  Create public and private keys:
+
+        # hackage-repo-tool create-keys \
+                            --keys ~/my-private-keys
+
+    This will create a directory structure such as
+
+        ~/my-private-keys/mirrors/id01.private
+        ~/my-private-keys/mirrors/..
+        ~/my-private-keys/root/id04.private
+        ~/my-private-keys/root/..
+        ~/my-private-keys/snapshot/id07.private
+        ~/my-private-keys/target/id08.private
+        ~/my-private-keys/target/..
+        ~/my-private-keys/timestamp/id11.private
+
+    containing keys for all the various TUF roles.
+
+    Note that these keys are stored outside of the repository proper.
+
+3.  Create the initial TUF metadata and construct an index using
+
+        # hackage-repo-tool bootstrap \
+                            --repo ~/my-secure-repo \
+                            --keys ~/my-private-keys
+
+    This will create a directory `~/my-secure-repo/index` containing the
+    `.cabal` files (extracted from the package tarballs) and TUF
+    metadata for all packages
+
+        ~/my-secure-repo/index/basic-sop/0.1.0.5/basic-sop.cabal
+        ~/my-secure-repo/index/basic-sop/0.1.0.5/package.json
+        ~/my-secure-repo/index/generics-sop/0.1.1.1/generics-sop.cabal
+        ~/my-secure-repo/index/generics-sop/0.1.1.1/package.json
+        ...
+
+    and package the contents of that directory up as the index tarball
+    `~/my-secure-repo/00-index.tar.gz`; it will also create the
+    top-level metadata files
+
+        ~/my-secure-repo/mirrors.json
+        ~/my-secure-repo/root.json
+        ~/my-secure-repo/snapshot.json
+        ~/my-secure-repo/timestamp.json
+
+4.  The timestamp and snapshot are valid for three days, so you will
+    need to resign these files regularly using
+
+        # hackage-repo-tool update \
+                            --repo ~/my-secure-repo \
+                            --keys ~/my-private-keys
+
+    You can use the same command whenever you add any additional
+    packages to your repository.
+
+5.  If you now make this directory available (for instance, by pointing
+    Apache httpd at it) you'll be able to use `cabal` to access it remotely by
+    defining an appropriate [`repository` stanza](https://www.haskell.org/cabal/users-guide/installing-packages.html#using-secure-repositories):
+
+        repository my-secure-repo
+          url: http://packages.example.org/
+          secure: True
+          root-keys: 2ae741f4c4a5f70ed6e6c48762e0d7a493d8dd265e9cbc6c4037dfc7ceaec70e
+                     32d3db5b4403935c0baf52a2bcb05031784a971ee2d43587288776f2e90609db
+                     eed36d2bb15f94628221cde558e99c4e1ad36fd243fe3748e1ee7ad00eb9d628
+          key-threshold: 2
+
+    Note that the keys in example above must be replaced: You need to
+    copy the root key IDs from your generated `root.json` file (or you
+    can set `key-threshold` to 0 if you're aware of the security
+    implications)