Docs: add DSO, GOT, and PLT to Lexicon.md

These terms are frequently referenced in Swift source code and comments. Having at least basic definitions for them in `Lexicon.md` should lower the barrier for new contributors.

Author: MaxDesiatov

docs/Lexicon.md

@@ -188,6 +188,12 @@ guarantee this. This was
 "Do not merge". Placed in PR titles where discussion or analysis is still
 ongoing.
 
+## DSO
+
+Dynamic shared object, an shared library file (.so/.dylib/.dll, the extension of
+which depends on the platform) to be used by multiple applications while they
+are executing.
+
 ## dup
 
 From "duplicate". As a noun, refers to another filed issue that describes
@@ -250,6 +256,15 @@ A representation of all generic parameters and their requirements. Like
 types, generic signatures can be [canonicalized](#canonical-type) to be
 compared directly.
 
+## GOT
+
+[Global offset table](https://en.wikipedia.org/wiki/Global_Offset_Table),
+a section in an executable or a shared library, which enables code to run
+independently of the memory address where it is loaded at runtime. Entries
+in GOT directly point to absolute addresses of symbols. The format of GOT
+is platform-dependent. Loaders update GOT relocations either on
+startup or on symbol access.
+
 ## iff
 
 ["if and only if"](https://en.wikipedia.org/wiki/If_and_only_if). This term comes from mathematics.
@@ -455,6 +470,12 @@ only to accelerate the process of reading C/C++/Objective-C headers, such as
 the bridging headers read in by the `-import-objc-header` command-line
 flag to swiftc.
 
+## PLT
+
+Procedure linkage table, which is used to call external functions that don't
+have their addresses known at link time. These addresses are then resolved
+by a loader at run time.
+
 ## PR
 
 1. "Problem Report": An issue reported in [LLVM's bug tracker](https://llvm.org/bugs/).