Updated documentation

- removed old CHANGELOG
- renamed README

Version 0.4.0

Poolable 

Built with GHC 8.2.2, 8.4.4, 8.6.5

According to RFC 4511 section 5.1:

The protocol elements of LDAP SHALL be encoded for exchange using the
Basic Encoding Rules [BER] of [ASN.1] with the following restrictions
[...]

The library does not have much choice except accepting everything the
server throws at it, therefore it makes sense using the more lax BER
encoding to parse server responses.

Using DER is still ok for the encoding part since DER is still a subset
of BER, and anything accepting BER will accept DER too.

implement SASL EXTERNAL authentication (tested with OpenLDAP and client-side certificates as the external auth)

Close #7.

`nix-build` can run tests without any problems now. Close #1.

Also, add many links to RFC, improve LDAPOID handling, and
other miscellaneous stuff.

Also improves the behavior when an `IOException` is encountered:
only exceptions directly related to LDAP are trapped by `with`.

  - Test `approximate` matches (even if they are mostly useless)

  - Partially support `Extensible` matching

  - Parse and ignore `SearchResultReference` responses, if any