added explanation for "squash commits first" error

Added a new documentation page that explains for
certain error messages why the error is occurring
and what can be done to solve the error.
As a first entry the error message "squash
commits first" is explained.

Signed-off-by: Edwin Kempin <edwin.kempin@gmail.com>
Change-Id: I60fb878610362f6930172db4ce534ac79745e10f

Documentation/error-messages.txt

diff --git a/Documentation/error-messages.txt b/Documentation/error-messages.txt
new file mode 100644
index 0000000..49c6c58
--- /dev/null
+++ b/Documentation/error-messages.txt
@@ -0,0 +1,17 @@
+Gerrit Code Review - Error Messages
+===================================
+
+This page provides access to detailed explanations of Gerrit error
+messages. For each error message it is explained why the error is
+occurring and what can be done to solve it.
+
+
+Error Messages
+--------------
+
+* link:error-squash-commits-first.html[squash commits first]
+
+
+GERRIT
+------
+Part of link:index.html[Gerrit Code Review]