developer-guide: Add openssl requirements and remove mbedtls one (fluent#948)

* developer-guide: Add openssl requirements and remove mbedtls one

Signed-off-by: Hiroshi Hatake <hatake@calyptia.com>
Co-authored-by: Pat <patrick.j.k.stephens@gmail.com>

Author: cosmo0920

development/developer-guide.md

@@ -8,7 +8,8 @@ Most external libraries are embedded in the project in the [/lib](https://github
 
 The external library you are mostly likely to interact with is [msgpack](https://github.com/msgpack/msgpack-c).
 
-For crypto, Fluent Bit uses [mbedtls](https://github.com/ARMmbed/mbedtls).
+For cryptographic support, Fluent Bit uses the system installed version of OpenSSL. 
+Please make sure to install openssl libraries and headers before building Fluent Bit.
 
 ### Memory Management