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

[mielvds]

Updates the README to the latest state of the repo and converts it to markdown (in line with other docs)

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	218	4	98%	27, 180, 565–566
context_resolver.py	103	8	92%	76, 135, 149, 195, 201–203, 227
iri_resolver.py	141	3	98%	307–308, 318
jsonld.py	2444	164	93%	271–275, 356, 393, 398, 408, 425–440, 476–477, 519, 527, 547, 555–556, 664–669, 749–750, 765–766, 826–831, 856–857, 872–873, 883–884, 910, 951, 961, 967–971, 982–983, 1016, 1025, 1032, 1097, 1120, 1142–1144, 1154–1157, 1172, 1195, 1207–1210, 1302, 1314–1317, 1337, 1366, 1399, 1403, 1440, 1483, 1502–1505, 1514, 1563, 1666–1673, 1700–1702, 1728–1731, 1907, 1945, 2353, 2362–2369, 2461, 2488, 2511, 2520, 3096, 3149, 3152, 3215, 3300, 3334, 3342, 3620, 3625, 3627, 3676, 3814–3818, 3884, 3921, 3984, 4219, 4507, 4641, 4675, 4704–4707, 4733–4734, 4917, 4981, 5008, 5082, 5084, 5102, 5200, 5249, 5481, 5483, 5522, 5524, 5533, 5672, 5800, 5887, 6007–6021, 6083, 6085, 6153, 6366, 6372–6376, 6415, 6418, 6508–6509, 6518
lib/pyld/documentloader
aiohttp.py	71	22	69%	29–39, 68, 74, 86, 98–106, 118–121, 138, 153–156
requests.py	49	5	90%	54, 68, 80–88
TOTAL	3214	206	94%

Tests	Skipped	Failures	Errors	Time
1687	50 💤	0 ❌	0 🔥	2m 52s ⏱️

[anatoly-scherbakov]

There are a few Markdown/RST conversion issues that affect rendered links. I left commit-able suggestions for the ones that are on changed diff lines.

One related issue is outside this PR diff: CHANGELOG.md still has an older README.rst#conformance link, which will be broken after this README rename.

[mielvds]

@anatoly-scherbakov thanks for patching those! I converted CONTRIBUTING as well so everything is in markdown now and changed the link in CHANGELOG. Feel free to have another quick look