Add a README.md file

README.md

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+The Automatic Certificate Management Environment (ACME), is an internet standard ([RFC 8555](https://tools.ietf.org/html/rfc8555)) which allows to automate X.509 certificates signing by a Certification Authority (CA). ACMEd is one of the many clients for this protocol.
+
+
+# Key features
+
+- HTTP-01 and DNS-01 challenges
+- RSA 2048, RSA 4096, ECDSA P-256 and ECDSA P-384 certificates
+- Fully customizable  challenge validation action
+- Run as a deamon: no need to set-up timers, crontab or other time-triggered process
+- Nice and simple configuration file
+
+
+# Build
+
+In order to compile ADMEd, you will need the latest stable version of [Rust](https://www.rust-lang.org/), although it should work with versions as low as 1.31.
+
+```
+cargo build --release
+```
+
+The executable is located in `target/release/acmed`.