Add security announce against 0.7.

wiki/src/security/Numerous_security_holes_in_0.7.mdwn

+[[!meta date="Thu Apr 28 11:12:13 2011"]]
+[[!meta title="Numerous security holes in Tails 0.7"]]
+
+[[!tag security/fixed]]
+
+The following security holes affect Tails 0.7. Some of these allow
+**remote arbitrary code execution with full administrator rights**,
+that can be exploited by any attacker with access to the local
+network.
+
+We **strongly** urge you to [[upgrade to Tails 0.7.1|news/version_0.7.1]]
+as soon as possible in case you are still using an older version.
+
+Details
+=======
+
+These are Debian security announces; details can be found on the
+[Debian security page](http://security.debian.org/):
+
+  - x11-xserver-utils (DSA-2213-1)
+  - isc-dhcp (DSA-2216-1)
+  - libmodplug (DSA-2226-1)