README.md: Point to our releases on pagure

Since the README.md is more or less what the wiki front page used to be,
it makes sense, especially for Github users, to point to our releases
from README.md

Reviewed-by: Lukáš Slebodník <[email protected]>

README.md

@@ -9,7 +9,19 @@ to connect to multiple different account sources.
 More information about SSSD can be found on its project page -
 https://pagure.io/SSSD/sssd/
 
-## Building and installation
+## Downloading SSSD
+SSSD is shipped as a binary package by most Linux distributions. If you
+want to obtain the latest source files, please navigate to the
+[Releases folder on pagure](https://releases.pagure.org/SSSD/sssd/)
+
+## Releases
+SSSD maintains two release streams - stable and LTM. Releases designated as
+LTM are long-term maintenance releases and will see bugfixes and security
+patches for a longer time than other releases.
+
+The list of all releases is maintained together with [SSSD documentation](https://docs.pagure.org/SSSD.sssd/users/releases.html)
+
+## Building and installation from source
 Please see the [our developer documentation](https://docs.pagure.org/SSSD.sssd/developers/)
 
 ## Documentation