Add license pointers for double-conversion, GMP and Rmath.

LICENSE.md

@@ -47,16 +47,19 @@ External libraries, if used, include their own licenses:
 
 - [ARPACK](http:https://www.caam.rice.edu/software/ARPACK/RiceBSD.txt#LICENSE)
 - [D3](https://github.com/mbostock/d3/raw/master/LICENSE)
+- [DOUBLE-CONVERSION](https://code.google.com/p/double-conversion/)
 - [DSFMT](http:https://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/LICENSE.txt)
 - [FDLIBM](http:https://www.netlib.org/fdlibm/fdlibm.h)
 - [FEMTOLISP](https://github.com/JeffBezanson/femtolisp)
 - [FFTW](http:https://fftw.org/doc/License-and-Copyright.html)
 - [GLPK](http:https://www.gnu.org/software/glpk)
+- [GMP](http:https://gmplib.org/manual/Copying.html#Copying)
 - [LAPACK](http:https://netlib.org/lapack/LICENSE.txt)
 - [LIBUNWIND](http:https://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=blob_plain;f=LICENSE;hb=master)
 - [LLVM](http:https://llvm.org/releases/3.0/LICENSE.TXT)
 - [OPENBLAS](https://raw.github.com/xianyi/OpenBLAS/master/LICENSE)
 - [PCRE](http:https://www.pcre.org/licence.txt)
 - [READLINE](http:https://cnswww.cns.cwru.edu/php/chet/readline/rltop.html)
+- [RMATH](http:https://www.r-project.org/Licenses/)
 - [SUITESPARSE](http:https://www.cise.ufl.edu/research/sparse/SuiteSparse/current/SuiteSparse/)
 - [ZLIB](http:https://zlib.net/zlib_license.html)