Start adding documentation for llvm-lib.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242557 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/index.rst

@@ -21,6 +21,7 @@ Basic Commands
    lli
    llvm-link
    llvm-ar
+   llvm-lib
    llvm-nm
    llvm-config
    llvm-diff

docs/CommandGuide/llvm-lib.rst

@@ -0,0 +1,31 @@
+llvm-lib - LLVM lib.exe compatible library tool
+===============================================
+
+
+SYNOPSIS
+--------
+
+
+**llvm-lib** [/libpath:<path>] [/out:<output>] [/llvmlibthin]
+[/ignore] [/machine] [/nologo] [files...]
+
+
+DESCRIPTION
+-----------
+
+
+The **llvm-lib** command is intended to be a ``lib.exe`` compatible
+tool. See https://msdn.microsoft.com/en-us/library/7ykb2k5f for the
+general description.
+
+**llvm-lib** has the following extensions:
+
+* Bitcode files in symbol tables.
+  **llvm-lib** includes symbols from both bitcode files and regular
+  object files in the symbol table.
+
+* Creating thin archives.
+  The /llvmlibthin option causes **llvm-lib** to create thin archive
+  that contain only the symbol table and the header for the various
+  members. These files are much smaller, but are not compatible with
+  link.exe (lld can handle them).