[llvm] [llvm][TableGen] Add a README to the main TableGen folder (PR #69943)

[Min-Yih Hsu via llvm-commits]

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@@ -0,0 +1,54 @@
+# LLVM TableGen
+
+The purpose of TableGen is to generate complex output files based on information
+from source files that are significantly easier to code than the output files would be, and also easier to maintain and modify over time.
+
+The information is coded in a declarative style involving classes and records,
+which are then processed by TableGen.
+
+```
+class Hello <string _msg> {
+	string msg = !strconcat("Hello ", _msg);
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mshockwave wrote:

nit: why is the indentation here different from that of line 19 and 23?

https://github.com/llvm/llvm-project/pull/69943