jtp: add really basic readme with motivation, etc

jtp/readme.md

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+# JTP: JSON Transfer Protocol
+
+JTP is a profile (in the same sense as RFC 3339) of HTTP/1.0 that facilitates extremely simple transfer of JSON objects or arrays over a TCP connection.
+
+## Motivation
+
+I had read about the `gemini://` protocol as a simplified alternative to `https://`, and so wanted to explore how complicated `https://` really was. Ultimately, I found that the subset of features needed to do a basic JSON transfer is extremely simple.
+
+## Request
+
+The format of the request I send is
+
+```
+GET /resource.json HTTP/1.0
+Host: server.org
+Accept: application/activity+json
+
+```
+
+I specify the version as `HTTP/1.0` to prevent the server from sending me a chunked-transfer encoded response. I specify `Host:` because, although not formally required in `HTTP/1.0`, many servers complain about its absence. `Accept:` is how I request that the server send me ActivityPub JSON instead of `text/html`.
+
+## Responses
+
+My response parsing handles any valid response to an `HTTP/1.0` request, but the most minimal responses I accept are as follows.
+
+In the successful case:
+
+```
+HTTP/1.1 200
+content-type: application/activity+json
+
+{
+    "the": "json"
+}
+```
+
+In the redirect case:
+
+```
+HTTP/1.1 300
+location: /over-here.json
+
+```
+
+And in the error case:
+
+```
+HTTP/1.1 400
+
+```