Documenting building against LLVM 3.8.

Fixes #8.

Author: Wilfred

CHANGELOG.md

@@ -4,6 +4,7 @@ Bug fixes:
 
 * Fixed an optimisation that incorrectly removed instructions when
   both `.` and `,` instructions were present.
+* Moved to LLVM 3.8, as LLVM 3.7 misoptimised some programs (see #8).
 
 Usability:
 

README.md

@@ -73,6 +73,33 @@ target triples:
 $ target/release/bfc sample_programs/hello_world.bf --target=x86_64-pc-linux-gnu
 ```
 
+### LLVM Version
+
+LLVM 3.8+ is recommended, as
+[there are known bugs with 3.7](https://github.com/Wilfred/bfc/issues/8). 3.8
+is not yet released, but you can use
+[the release candidate](http://llvm.org/pre-releases/3.8.0/). Either
+download a prebuilt LLVM, or build it as follows:
+
+```
+$ wget http://llvm.org/pre-releases/3.8.0/rc1/llvm-3.8.0rc1.src.tar.xz
+$ tar -xf llvm-3.8.0rc1.src.tar.xz
+
+$ mkdir -p ~/tmp/llvm_3_8_build
+$ cd ~/tmp/llvm_3_8_build
+
+$ cmake -G Ninja /path/to/untarred/llvm
+$ ninja
+```
+
+bfc depends on llvm-sys, which compiles against whichever
+`llvm-config` it finds.
+
+```
+$ export PATH=~/tmp/llvm_3_8_build:$PATH
+$ cargo build --release
+```
+
 ### Running tests
 
 ```