Write a change log from mbedcrypto-2.0.0 to the merge of #75

Get started on writing a change log file for Mbed Crypto. I went
through pull requests merged since the tag mbedcrypto-2.0.0 and up
to #75, i.e. commit 9ab7c07f1f.

ChangeLog.md

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+# Mbed Crypto change history
+
+## Unreleased changes
+
+### Interface changes
+
+* In the PSA API, forbid zero-length keys. To pass a zero-length input to a key derivation function, use a buffer instead (this is now always possible).
+
+### New features
+
+* Key derivation inputs in the PSA API can now either come from a key object or from a buffer regardless of the step type.
+
+### Bug fixes
+
+* Fix a buffer overflow in the PSA HMAC code when using a long key with an unsupported algorithm. Fixes #254.
+* Fix `mbedtls_asn1_get_int` to support any number of leading zeros.
+* Fix `mbedtls_asn1_get_bitstring_null` to correctly parse bitstrings of at most 2 bytes.
+
+### Performance improvements
+
+* Remove a useless call to mbedtls_ecp_group_free(). Contributed by Alexander Krizhanovsky in #210.
+* Speed up PBKDF2 by caching the digest calculation. Contributed by Jack Lloyd and Fortanix Inc in #277.
+
+### Other changes
+
+* Remove the technical possibility to define custom md_info structures, which was exposed only in an internal header.
+
+## Mbed Crypto 2.0.0