Allow using mkl in tests. (#90)

README.md

@@ -1,2 +1,23 @@
 # candle
 Minimalist ML framework for Rust
+
+## FAQ
+
+### Missing symbols when compiling with the mkl feature.
+
+If you get some missing symbols when compiling binaries/tests using the mkl
+features, e.g.:
+```
+  = note: /usr/bin/ld: (....o): in function `blas::sgemm':
+          .../blas-0.22.0/src/lib.rs:1944: undefined reference to `sgemm_' collect2: error: ld returned 1 exit status
+
+  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
+  = note: use the `-l` flag to specify native libraries to link
+  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-link-libkindname)
+```
+
+This is likely due to some missing linker flag that enable the mkl library. You
+can try adding the following at the top of your binary:
+```
+extern crate intel_mkl_src;
+```