add BUILD.md file that explains how to do complex builds

BUILD.md

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+# Buil the project on your machine
+
+Cargo is the default manager to build rust project and this particular project is not an exception.
+
+## Bindgen
+
+Normally no other dependencies are required outside Cargo itself, however, we use [`bindgen`][bindgen] to automatically create the FFI binds between the C code from Redis and SQLite and the Rust code.
+
+At the moment we have decided to keep generating the bindings during compilation, this is not strictly necessary, but at the moment it seems the best alternative.
+
+`bindgen` has as dependencies: `llvm` and `clang` that need to be installed in your machine.
+Please refer to the `bindgen` [documentation][bindgen_dependencies] in order to understand how to install the dependencies, it is quite easy in modern systems.
+
+## GLIBC
+
+Rust usually try to link statically as much as possible, however, leave libc dynamically linked.
+
+Usually, it is not a problem, but this requires that the machine where the object will execute has at least the version of glibc assumed during compilation.
+
+If your servers are running relatively modern operative systems this problem will never affect you, however, if you are running more "battle tested" OSs, then you may encounter these difficulties.
+
+We decided to support GLIBC 2.14 that should cover the vast majority of used operative systems with the notable exception of CentOS 6, if there is enough request for CentOS 6 we will drop to GLIBC 2.12
+
+If you are building the module yourself, and if you need a lower GLIBC version than the one automatically used by cargo you can use [crosstool-ng][x-tool]:
+
+1. Configure your toolchain: `./ct-ng menuconfig`
+
+Most likely you will need to select `Use obsolete features` under `Paths and misc options` and then choose the glibc version you need under `c-library`
+
+2. Build your toolchain (it will take a while ~30min/1 hour): `./ct-ng build`
+
+3. Configure the rustc linker in such a way to use the one you have just build.
+
+In my case it looks a little bit like this:
+
+```
+RUSTFLAGS="-C linker=~/x-tools/x86_64-unknown-linux-gnu/bin/x86_64-unknown-linux-gnu-gcc" cargo build --release 
+```
+
+Beware that your path could be a little different.
+
+## Conclusion
+
+In the general case, it is quite simple to build RediSQL, however specific cases need to be addressed with more attention.
+
+Please, if you find any issues following this instruction we would really appreciate you reporting them via github issues or pull requests. 
+
+
+[bindgen]: https://rust-lang-nursery.github.io/rust-bindgen/introduction.html
+[bindgen_dependencies]: https://rust-lang-nursery.github.io/rust-bindgen/requirements.html
+[x-tool]: https://crosstool-ng.github.io/