Add README.md to arch/inst (#531)

Co-authored-by: Your Name <[email protected]>
Co-authored-by: Derek Hower <[email protected]>

Author: 3 people

arch/inst/README.md

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+# Instructions Repository
+
+This repository contains definitions for instructions following the standardized JSON schema provided by UDB (Universal Design Base). Each instruction is specified within its own YAML file.
+
+## Structure
+
+Instructions are organized under the following path:
+
+```
+arch/
+  inst/
+    <extension_name>/ # Typically alphabetical order of extensions defining instructions; however, this is not mandatory.
+      <instruction_name>.yaml
+```
+
+## Creating an Instruction Definition
+
+Each instruction definition should conform to the provided schema (`inst_schema.json`). Below is a detailed template and explanation for creating a new instruction YAML file.
+
+### YAML Template
+
+```yaml
+$schema: "inst_schema.json#"
+kind: instruction
+name: <instruction_mnemonic>
+long_name: <Brief description of the instruction>
+description: |
+  Detailed explanation of what the instruction does.
+  Use clear and precise asciidoc-formatted text.
+definedBy: <Extension defining this instruction>
+assembly: <assembly_format>
+encoding:
+  match: <binary encoding using 0, 1, and ->
+  variables:
+    - name: <variable_name>
+      location: <bit range, e.g., 24-20>
+access:
+  s: always | sometimes | never
+  u: always | sometimes | never
+  vs: always | sometimes | never
+  vu: always | sometimes | never
+data_independent_timing: true | false
+operation(): |
+  # Optional IDL operation
+  leave empty if not provided
+
+sail(): |
+  # Optional Sail operation
+  leave empty if not provided
+```
+
+### Explanation of Fields
+
+- **`name`**: The mnemonic of the instruction (lowercase, alphanumeric and periods only).
+- **`long_name`**: Short, human-readable description.
+- **`description`**: Full, detailed description of the instruction behavior (asciidoc format).
+- **`definedBy`**: Specifies the extension defining this instruction.
+- **`assembly`**: Instruction format in assembly language, including operands.
+- **`encoding.match`**: Binary encoding of the instruction with fixed bits defined by `0` or `1` and variable bits indicated by `-`.
+- **`encoding.variables`**: Defines fields in the instruction encoding, including their location.
+  - **`name`**: Name of the field (e.g., `rs1`, `rs2`).
+  - **`location`**: Bit positions of the field in the instruction encoding.
+- **`access`**: Specifies the privilege mode access for the instruction (`always`, `sometimes`, or `never`).
+- **`data_independent_timing`**: Indicates whether the execution timing is data-independent.
+- **`operation()` & `sail()`**: Optional fields for IDL or Sail descriptions. Leave empty if unused.
+
+## JSON Schema
+
+All instruction definitions must adhere strictly to the provided JSON schema:
+
+[`inst_schema.json`](schemas/inst_schema.json)
+
+Ensure compliance with schema validation to facilitate integration and usage within UDB.