Update stackoverflow.com install counters

There's ~27k questions by now, not just 7k  :wink:

Signed-off-by: Fabian Lauer <kontakt@softwareschmiede-saar.de>

README.md & CONTRIBTUTING.md: Use word `thousand` instead of numbers

27000 -> 27 thousand

Signed-off-by: Fabian Lauer <kontakt@softwareschmiede-saar.de>

README+CONTRIB: No specific number of SO questions

Signed-off-by: Fabian Lauer <kontakt@softwareschmiede-saar.de>

CONTRIBUTING.md: Remove unnecessary word

Signed-off-by: Fabian Lauer <kontakt@softwareschmiede-saar.de>

CONTRIBUTING.md

@@ -137,7 +137,7 @@ However, there might be a way to implement that feature *on top of* Docker.
   <tr>
     <td>Stack Overflow</td>
     <td>
-      Stack Overflow has over 17000 Docker questions listed. We regularly
+      Stack Overflow has thousands of Docker questions listed. We regularly
       monitor <a href="https://stackoverflow.com/search?tab=newest&q=docker" target="_blank">Docker questions</a>
       and so do many other knowledgeable Docker users.
     </td>

README.md

@@ -252,7 +252,7 @@ We are always open to suggestions on process improvements, and are always lookin
   <tr>
     <td>Stack Overflow</td>
     <td>
-      Stack Overflow has over 7000 Docker questions listed. We regularly
+      Stack Overflow has thousands of Docker questions listed. We regularly
       monitor <a href="https://stackoverflow.com/search?tab=newest&q=docker" target="_blank">Docker questions</a>
       and so do many other knowledgeable Docker users.
     </td>