Fix links to documentation in README.md (#404)

Signed-off-by: Arthur Pitman <[email protected]>

README.md

@@ -65,10 +65,10 @@ As highlighted above, the *dynatrace-service* also provides the following capabi
 (*) *Note:* 0.11.0 is feature-complete with 0.10.0. Changes and fixes made from 0.10.1 to 0.10.3 will be incorporated in 0.12.0
 
 ## Overview
-- [Installation](./documentation/installation)
-- [Configuration](./documentation/configuration)
-- [SLI and SLO configuration](./documentation/sli-configuration)
-- [Development](./documentation/development)
+- [Installation](./documentation/installation.md)
+- [Configuration](./documentation/configuration.md)
+- [SLI and SLO configuration](./documentation/sli-configuration.md)
+- [Development](./documentation/development.md)