Add build instructions to the README.md.

README.md

@@ -1,5 +1,53 @@
-This implements the [p≡p Engine]'s [cryptotech] interface using [Sequoia].
+This library provides an implementation of the [p≡p Engine]'s
+[cryptotech] interface using [Sequoia].
 
   [p≡p Engine]: https://gitea.pep.foundation/pEp.foundation/pEpEngine
   [cryptotech]: https://gitea.pep.foundation/pEp.foundation/pEpEngine/src/branch/master/src/cryptotech.h
   [Sequoia]: https://sequoia-pgp.org
+
+Building
+========
+
+You need at least version 1.48 of `rustc` and `cargo`.
+
+You can build this library as follows:
+
+```
+$ git clone https://gitea.pep.foundation/neal/pEpEngineSequoiaBackend.git
+$ cd pEpEngineSequoiaBackend
+$ mkdir -p /tmp/pep_engine_sequoia_backend
+$ CARGO_TARGET_DIR=/tmp/pep_engine_sequoia_backend cargo build # Add --release for a release build
+$ CARGO_TARGET_DIR=/tmp/pep_engine_sequoia_backend cargo test
+```
+
+This will generate, among others
+`/tmp/pep_engine_sequoia_backend/debug/pep_engine_sequoia_backend.pc`.
+This can be used to easily link to the library *in place*.  That is,
+no installation is required.
+
+Hence to build and test the engine, we can do:
+
+```
+$ cd ~/src/pEpEngine
+$ export PKG_CONFIG_PATH=/tmp/pep_engine_sequoia_backend/debug${PKG_CONFIG_PATH+:$PKG_CONFIG_PATH}
+$ make
+$ LD_LIBRARY_PATH=$(for p in $(pkg-config pep_engine_sequoia_backend --libs-only-L); do echo ${p#-L}; done | paste -sd ':')${LD_LIBRARY_PATH+:$LD_LIBRARY_PATH} RUST_BACKTRACE=1 make -C test test
+```
+
+Of course when installing the engine, we'll need to distribute the
+generated library.
+
+Note: when profiling the library (or doing a release), be sure to
+build in release mode!  That is, build the library with `cargo build
+--release` and replace `debug` with `release` in the second set of
+commands.
+
+When built in debug mode, the library always generates trace output.
+If you encounter problems in release mode, you can get a trace of the
+library's execution by setting the `PEP_TRACE` environment
+variable. For example:
+
+```
+$ cd ~/src/pEpEngine/test
+$ PKG_CONFIG_PATH=/tmp/pep_engine_sequoia_backend/debug${PKG_CONFIG_PATH+:$PKG_CONFIG_PATH} LD_LIBRARY_PATH=$(for p in $(pkg-config pep_engine_sequoia_backend --libs-only-L); do echo ${p#-L}; done | paste -sd ':')${LD_LIBRARY_PATH+:$LD_LIBRARY_PATH} PEP_TRACE=1 ./EngineTests -- --gtest_filter=DeleteKeyTest.check_delete_single_pubkey
+```