Drop requirement for a CLA

The Mbed TLS project no longer requires a CLA. Contributions from now on
must be made under both Apache-2.0 AND GPL-2.0-or-later licenses, to enable
LTS (Long Term Support) branches of the software to continue to be provided
under either Apache-2.0 OR GPL-2.0-or-later. Contributors must accept the
terms of the Developer Certificate of Origin (DCO) by adding a Signed-off-by:
line to each commit message.

The software on the development branch continues to be provided under
Apache-2.0.

Update README.md, CONTRIBUTING.md and pull_request_template.md to explain
the new licensing model. Add a copy of the DCO to the project.

Expand the full Apache-2.0 license text in the LICENSE file and remove the
redundant apache-2.0.txt.

Signed-off-by: Dan Handley <dan.handley@arm.com>
diff --git a/dco.txt b/dco.txt
new file mode 100644
index 0000000..8201f99
--- /dev/null
+++ b/dco.txt
@@ -0,0 +1,37 @@
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.