Add project description and development documentation

README.md

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+# CAcert signer client
+
+This is the CAcert signer client implementation that sends commands to the signer and handles certificate
+revocation lists (CRLs), OpenPGP public key signatures and X.509 certificates.
+
+## Development preconditions
+
+You will need GNU make to build the application. On Debian systems you can install GNU make using:
+
+```shell
+sudo apt install make
+```
+
+Install [go](https://go.dev/) >= 1.17, [golangci-lint](https://golangci-lint.run/usage/install/) >= 1.50.0 and
+[goreleaser](https://goreleaser.com/install/), as these are used for building and linting the application.
+
+Read the documentation of these tools, to find out how to use them.
+
+## Building the binary
+
+There is a `Makefile` to build the signerclient binary. Run
+
+```shell
+make
+```
+
+to run linting, tests and binary builds.