Update instructions:

1. git clone https://boringssl.googlesource.com/boringssl
2. git checkout <commithash> (currently 82aa28fa)
3. Build BoringSSL according to its instructions
4. mkdir -p third_party/boringssl/src
5. cp -R <BoringSSL source tree>/* third_party/boringssl/src
6. cp <BoringSSL source tree>/build/crypto/err/err_data.c third_party/boringssl
7. Done.