doc: python bindings howto

* Added c.get_key instructions and examples.

lang/python/docs/GPGMEpythonHOWTOen.org

@@ -345,7 +345,7 @@
    #+end_src
 
 
-** Counting keys
+*** Counting keys
     :PROPERTIES:
     :CUSTOM_ID: howto-keys-counting
     :END:
@@ -375,6 +375,49 @@
     #+end_src
 
 
+** Get key
+   :PROPERTIES:
+   :CUSTOM_ID: howto-get-key
+   :END:
+
+   An alternative method of getting a single key via its fingerprint
+   is available directly within a Context with =Context().get_key=.
+   This is the preferred method of selecting a key in order to modify
+   it, sign or certify it and for obtaining relevant data about a
+   single key as a part of other functions; when verifying a signature
+   made by that key, for instance.
+
+   By default this method will select public keys, but it can select
+   secret keys as well.
+
+   This first example demonstrates selecting the current key of Werner
+   Koch, which is due to expire at the end of 2018:
+
+   #+begin_src python
+     import gpg
+
+     fingerprint = "80615870F5BAD690333686D0F2AD85AC1E42B367"
+     key = gpg.Context().get_key(fingerprint)
+   #+end_src
+
+   Whereas this example demonstrates selecting the author's current
+   key with the =secret= key word argument set to =True=:
+
+   #+begin_src python
+     import gpg
+
+     fingerprint = "DB4724E6FA4286C92B4E55C4321E4E2373590E5D"
+     key = gpg.Context().get_key(fingerprint, secret=True)
+   #+end_src
+
+   It is, of course, quite possible to select expired, disabled and
+   revoked keys with this function, but only to effectively display
+   information about those keys.
+
+   It is also possible to use both unicode or string literals and byte
+   literals with the fingerprint when getting a key in this way.
+
+
 * Basic Functions
   :PROPERTIES:
   :CUSTOM_ID: howto-the-basics