docs: update installation instructions

feat: support passing http.Header to exported functions (#37)

tika: implement Parse and Translate methods that return readers (#34)

Always reading the response body into memory results in unnecessary and
undesired allocations, especially when sending large documents to Tika
for parsing.

The call method, which is called for all API requests, doesn't copy the
response anymore. Instead, callString copies it for the methods that
return strings and callUnmarshal does its own buffering using the JSON
decoder.

This commit also creates a new method, ParseReader, which allows users
of the client to omit copying the response, but instead receive its
reader directly, which they are responsible to close.

As with ParseReader, a TranslateReader is provided for the same reasons.

all: add support for 1.20 and 1.21

Fix typo in readme