add an example with / in namespace to README and SPEC

I think it is not clear from the text in the README and the examples
whether a `/` in the namespace needs to be escaped. This adds an example
from `/test-suite-data.json` to the list of examples, to make that
clear, that escaping should not be done.

A follow-up question, that is not scope of this PR, would be:
=============================================================
given the PURL `pkg:swift/github.com%2FAlamofire/Alamofire@5.4.3`
is `pkg:swift/github.com/Alamofire/Alamofire@5.4.3` its canonical form?
If yes, it should be added to the test cases.

Or, for having more fun and with looking at package-url#63 : what is the canonical
form of `pkg:golang/github.com%2Frussross%2Fblackfriday%2Fv2@v2.1.0`?

Signed-off-by: Maximilian Huber <gh@maxhbr.de>

PURL-SPECIFICATION.rst

@@ -70,6 +70,7 @@ Some `purl` examples
     pkg:nuget/EnterpriseLibrary.Common@6.0.1304
     pkg:pypi/django@1.11.1
     pkg:rpm/fedora/curl@7.50.3-1.fc25?arch=i386&distro=fedora-25
+    pkg:golang/github.com/gorilla/context@234fd47e07d1004f0aed9c
 
 
 A `purl` is a URL

README.rst

@@ -136,6 +136,8 @@ Some `purl` examples
     pkg:rpm/fedora/curl@7.50.3-1.fc25?arch=i386&distro=fedora-25
     pkg:rpm/opensuse/curl@7.56.1-1.1.?arch=i386&distro=opensuse-tumbleweed
 
+    pkg:swift/github.com/Alamofire/Alamofire@5.4.3
+
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