Convert README AND CONTRIBUTING from rst to md and update the contents.

CHANGELOG.md

@@ -16,6 +16,7 @@
   class-based `DocumentLoader` instances while preserving the existing
   callable factory API. The concrete `RequestsDocumentLoader` and
   `AioHttpDocumentLoader` classes are also importable from `pyld`.
+- Convert `./README.rst` (reStructuredText) to `./README.md` (markdown) and update its contents. 
 
 ### Added
 - `pyld.DocumentLoader` abstract base class for class-based document loaders,

CONTRIBUTING.rst

@@ -78,7 +78,7 @@ As of early 2020, the process to generate an EARL report for the official
 `JSON-LD Processor Conformance`_ page is:
 
 * Run the tests on the ``json-ld-api`` and ``json-ld-framing`` test repos to
-  generate a ``.jsonld`` test report as explained in [README.rst](./README.rst#tests)
+  generate a ``.jsonld`` test report as explained in [README.md](./README.md#tests)
 * Use the rdf_ tool to generate a ``.ttl``:
 
   * ``rdf serialize pyld-earl.jsonld --output-format turtle -o pyld-earl.ttl``

MANIFEST.in

@@ -1,2 +1,2 @@
-include README.rst README.txt LICENSE CHANGELOG.md
+include README.md README.txt LICENSE CHANGELOG.md
 recursive-include lib/pyld/documentloader/frozen/bundled *.jsonld