CIPHERS.md: document GSKit ciphers

`ecdhe_rsa_chacha20_poly1305_sha_256`

`ecdhe_ecdsa_chacha20_poly1305_sha_256`

`dhe_rsa_chacha20_poly1305_sha_256`

## GSKit

Ciphers are internally defined as numeric codes (http://www.ibm.com/support/knowledgecenter/ssw_ibm_i_73/apis/gsk_attribute_set_buffer.htm),

but libcurl maps them to the following case-insensitive names.

### SSL2 cipher suites (insecure: disabled by default)

`rc2-md5`

`rc4-md5`

`exp-rc2-md5`

`exp-rc4-md5`

`des-cbc-md5`

`des-cbc3-md5`

### SSL3 cipher suites

`null-md5`

`null-sha`

`rc4-md5`

`rc4-sha`

`exp-rc2-cbc-md5`

`exp-rc4-md5`

`exp-des-cbc-sha`

`des-cbc3-sha`

### TLS v1.0 cipher suites

`null-md5`

`null-sha`

`rc4-md5`

`rc4-sha`

`exp-rc2-cbc-md5`

`exp-rc4-md5`

`exp-des-cbc-sha`

`des-cbc3-sha`

`aes128-sha`

`aes256-sha`

### TLS v1.1 cipher suites

`null-md5`

`null-sha`

`rc4-md5`

`rc4-sha`

`exp-des-cbc-sha`

`des-cbc3-sha`

`aes128-sha`

`aes256-sha`

### TLS v1.2 cipher suites

`null-md5`

`null-sha`

`null-sha256`

`rc4-md5`

`rc4-sha`

`des-cbc3-sha`

`aes128-sha`

`aes256-sha`

`aes128-sha256`

`aes256-sha256`

`aes128-gcm-sha256`

`aes256-gcm-sha384`