Merge bitcoin#703: Overhaul README.md

2e759ec Overhaul README.md (Tim Ruffing)

Pull request description:

  * Update feature list
  * Be more positive about the state and quality of the library
  * Mention ECDSA key operations explicitly in short library description
  * Say "secret key" instead of "private key"

  cc @gmaxwell who suggested a similar wording for the disclaimer.

ACKs for top commit:
  sipa:
    ACK 2e759ec
  jonasnick:
    ACK 2e759ec

Tree-SHA512: 2e1c87e7fa28d9dab682af227f845e7d48ac79a9fbe10be47ae4567abc2e066ba2f852c000db7d697ece8e4bbeeb851ea647465f870ac29dc3654031bf15a1ad

README.md

@@ -3,17 +3,22 @@ libsecp256k1
 
 [![Build Status](https://travis-ci.org/bitcoin-core/secp256k1.svg?branch=master)](https://travis-ci.org/bitcoin-core/secp256k1)
 
-Optimized C library for EC operations on curve secp256k1.
+Optimized C library for ECDSA signatures and secret/public key operations on curve secp256k1.
 
-This library is a work in progress and is being used to research best practices. Use at your own risk.
+This library is intended to be the highest quality publicly available library for cryptography on the secp256k1 curve. However, the primary focus of its development has been for usage in the Bitcoin system and usage unlike Bitcoin's may be less well tested, verified, or suffer from a less well thought out interface. Correct usage requires some care and consideration that the library is fit for your application's purpose.
 
 Features:
 * secp256k1 ECDSA signing/verification and key generation.
-* Adding/multiplying private/public keys.
-* Serialization/parsing of private keys, public keys, signatures.
-* Constant time, constant memory access signing and pubkey generation.
-* Derandomized DSA (via RFC6979 or with a caller provided function.)
+* Additive and multiplicative tweaking of secret/public keys.
+* Serialization/parsing of secret keys, public keys, signatures.
+* Constant time, constant memory access signing and public key generation.
+* Derandomized ECDSA (via RFC6979 or with a caller provided function.)
 * Very efficient implementation.
+* Suitable for embedded systems.
+* Optional module for public key recovery.
+* Optional module for ECDH key exchange (experimental).
+
+Experimental features have not received enough scrutiny to satisfy the standard of quality of this library but are made available for testing and review by the community. The APIs of these features should not be considered stable.
 
 Implementation details
 ----------------------
@@ -28,7 +33,7 @@ Implementation details
 * Field operations
   * Optimized implementation of arithmetic modulo the curve's field size (2^256 - 0x1000003D1).
     * Using 5 52-bit limbs (including hand-optimized assembly for x86_64, by Diederik Huys).
-    * Using 10 26-bit limbs.
+    * Using 10 26-bit limbs (including hand-optimized assembly for 32-bit ARM, by Wladimir J. van der Laan).
   * Field inverses and square roots using a sliding window over blocks of 1s (by Peter Dettman).
 * Scalar operations
   * Optimized implementation without data-dependent branches of arithmetic modulo the curve's order.
@@ -50,7 +55,7 @@ Implementation details
     * Access the table with branch-free conditional moves so memory access is uniform.
     * No data-dependent branches
   * Optional runtime blinding which attempts to frustrate differential power analysis.
-  * The precomputed tables add and eventually subtract points for which no known scalar (private key) is known, preventing even an attacker with control over the private key used to control the data internally.
+  * The precomputed tables add and eventually subtract points for which no known scalar (secret key) is known, preventing even an attacker with control over the secret key used to control the data internally.
 
 Build steps
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