readme: Add CounterType and ProgrammingRestriction

[edwarddavidbaker]

Add documentation to describe event fields CounterType and ProgrammingRestriction.

Pre-merge rendered markdown can be viewed at https://github.com/edwarddavidbaker/perfmon/tree/readme-updates#countertype .

[edwarddavidbaker]

Thanks for drafting readme content Ryan. What do you think about the [N] indexing? Do you think adding a small mermaid diagram would help succinctly show an example?

[rpmclaug]

Suggested change

	there are several `UMask` entries, then `UMask[N]` and `MSRIndex[N]` are used to select which UMask to program, and
	which MSR is written. Any of the `MSRIndex` entries is compatible with counters listed in counter fields.
	there are multiple `UMask` entries, then `UMask` and `MSRIndex` should be used paired in their specified order.
	An event configured with the `N`th UMask, `UMask[N]` must write `MSRValue` to the MSR indicated at the `N`th MSRIndex, `MSRIndex[N]` .
	Any of the `MSRIndex` entries is compatible with counters listed in counter fields.

[rpmclaug]

@edwarddavidbaker Ideally this document could remain legible in text editors; If there's a means to clarify the text description, let's start there.

Can you provide an example of the sort of diagram you think would help?

One possibility to help differentiate these:

flowchart

    MUX(MSRIndex-UMask
    UMask: A,B
    MSRIndex: C,D
    Counter: 0,1) --> MUY{UMask: A
    MSRIndex: C
    Counter: 0,1}
    MUX --> MUZ{UMask: B
    MSRIndex: D
    Counter: 0,1}

    MUCX(MSRIndex-UMask-Counter
    UMask: A,B
    MSRIndex: C,D
    Counter: 0,1) --> MUCY{UMask: A
    MSRIndex: C
    Counter: 0}
    MUCX --> MUCZ{UMask: B
    MSRIndex: D
    Counter: 1}

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