Docs: install from source, verify signature

Add chapter for installing from source with signature verification.
Resolves #795.

docs/changelog

@@ -7,6 +7,7 @@ next:
 	* Documentation: Embed config skeleton instead of rewriting it (Darko Poljak)
 	* Documentation: Remove cdist-type prefix and man page reference from type list in html (Darko Poljak)
 	* Documentation: PreOS english nitpicking (Evil Ham)
+	* Documentation: Add installing from source with signature verification (Darko Poljak)
 
 6.2.0: 2019-11-30
 	* Core: Redefine/reimplement/fix CDIST_ORDER_DEPENDENCY (Darko Poljak)

docs/src/cdist-install.rst

@@ -23,8 +23,6 @@ Target Hosts
 Install cdist
 -------------
 
-You can install cdist either from git or as a python package.
-
 From git
 ~~~~~~~~
 
@@ -42,6 +40,7 @@ To install cdist, execute the following commands:
 
 From version 4.2.0 cdist tags and releases are signed.
 You can get GPG public key used for signing `here <_static/pgp-key-EFD2AE4EC36B6901.asc>`_.
+It is assumed that you are familiar with *git* ways of signing and verification.
 
 You can also get cdist from `github mirror <https://github.com/ungleich/cdist>`_.
 
@@ -157,3 +156,27 @@ Cdist is available as a python package at
 .. code-block:: sh
 
     pip install cdist
+
+Installing from source with signature verification
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you want to install cdist from signed source and verify it, first you need to
+download cdist archive and its detached signature.
+
+Get both, *cdist-x.y.z.tar.gz* and *cdist-x.y.z.tar.gz.asc* from release
+notes of the desired tag *x.y.z* at
+`cdist git repository <https://code.ungleich.ch/ungleich-public/cdist/-/tags>`_.
+
+Get GPG public key used for signing `here <_static/pgp-key-EFD2AE4EC36B6901.asc>`_
+and import it into GPG.
+
+Now cdist source archive can be verified using `gpg`, e.g. to verify `cdist-6.2.0`:
+
+.. code-block:: sh
+
+    $ gpg --verify cdist-6.2.0.tar.gz.asc cdist-6.2.0.targ.gz
+    gpg: Signature made Sat Nov 30 23:14:19 2019 CET
+    gpg:                using RSA key 69767822F3ECC3C349C1EFFFEFD2AE4EC36B6901
+    gpg: Good signature from "ungleich GmbH (ungleich FOSS) <foss@ungleich.ch>" [ultimate]
+
+Further steps are the same as for `installing from git <cdist-install.html#from-git>`_.